Materials for the Study of Variation Treated with Especial Regard to Discontinuity in the Origin of Species

one, presumably the supernumerary, stands almost exactly in the middle line. This tooth is turned half round, so that the plane of its chisel stands obliquely. The teeth are all well formed and none belong to the milk -dentition, for the milk-teeth are much smaller and of different form. I did not succeed in satisfying myself that the central tooth is certainly the supernumerary. The second incisors are in place on each side and are quite distinct, and the right first incisor is similarly normal.

lint whether the nbliijuu tooth, or the tooth between it and tinri^ht /', should I).- rather consiflered supernumerary cannot be deel.uvd \vith certainty. Probably this is one of the cases, of \\hieh more will be said hereafter, in which both teeth replace the normally single il. C. S. M. 21, A. txtru ti'ftli. A lower jaw in the Museum of the Odontological . ty "hav in;.- two supernumerary teeth embedded in the bone beneath the coro- ; -moid IK. tdi. Originally only a small nodule of enamel was

• le nn tli.. inner surface nf the right ascending ramus, just external to the upper extremity of tlif inf.-ri.ir dental canal. On cutting away the bone this nodule was fiMiii'l i» }»• a portion of a supernumerary tooth having a conical crown and a •le taper. Lying above it, another supernumerary tooth was discovered, of which there had previously been no sign whatever. This was likewise exposed i«y removing the superjarent bone, and found to be a larger tooth with a conical

.11 iiii'l three Ion;,- narrow roots. The teeth were lying parallel to each other, with their crownpointing upwards and backwards, so that they could hardly under uny ( in iuii-[:tiK e- h;i\r been erupted in the alveolar arch." Trans. Odont. Soc., L887, nx. p. •-'••.»;. ;ig. Sp. einien having fragment of a tooth imbedded in bone between left lower canine and p1; perhaps a fragment of a milk-tooth P.M., A, 500. [Two specimen-; in the stores of the P.M. shew great irregularities in the arrangement of the teeth; but in both cases so many teeth had been lost during life that u satisfactory description of the abnormalities cannot now be given.]

Of tinin 'in T;I tii'iiiitujn'tJieciis, Colobus, Nasalis, Cercopithecus, Oercocebus, Mm-ur,/* arid Cynocephalus ; 41!) normal specimens examined. Only two had definite supernumerary teeth, but in one «>ther case it was possible that extra molars had been present. Cynocephalus porcarius, having large extra molar behind and in series in each upper jaw. The two teeth are of the same pattern precisely. In lower jaw there is on each side a large behind /// ', but there is no tooth in it. O. M., 2011, b.

1!M). Macacus rhesus, old male, having a fourth molar in place in ri^ht luin-r jaw. The tooth does not stand up fully from the bone. On the same side in the iij>/n'r jaw there is also a fourth molar, but was entirely enclosed in bone and was only found by eiittinir a\\av the side of the maxilla bv way of exploration. B.M., 30, c. 191. Macacus radlatus, basing small and fairly definite depression behind >»:t in • aeli jaw. These depressions seem to tie peihaps the alveoli of teeth but it cannot be p" iti'.eU atated that extra molars have been present. C.S.M., 145.

I|i2. .tl-iinriiiiil (irr,n\<ii-M<-nt. Only one case of considerable irregularity of arrangement seen, vi/.., Ccrcopitbccus lalandii (C. S. M. 1 l.li, case in which lower canines are recurved and pass l>,-himl the upper ones, bee Cut. Jl/«x. Coll. S *193. In the species of Cebidre and especially in Ateles supernumerary teeth are rather common, eight cases being found in 28-t skulls, or nearly 3 per cent, (in addition to cases recorded by others). Of American monkeys belonging to other genera 92 skulls were seen, all being normal. Some cases of absence of the third molar were seen in Ateles, which are interesting in connexion with the fact that there are normally only two molars in Hapalidse.

194. Cebus robustus : supernumerary molar in each upper jaw giving p f , m f ; DE BLAINVILLE, Laurent's Annal. d'Anat. etPhys., 1837, i. p. 300, PL VIIL fig. 6. 19*5. c. variegatus : small tubercular molar in right lower jaw behind ^. The extra tooth is cylindrical and peg-like, having about ^th the diameter m?. Leyd. Mus. 8, Cat. 11. Ateles : normal adult skulls, belonging to several species, 60. *196. A. pentadactylus : extra molar in series behind m3 in both upper and lower jaws on right side, in each case a small round tooth. P. M., A, 1505. This specimen described by DE BLAINVILLE Laurent's Ann. d'Anat. et Phys., 1837, I. p. 300, PI. VIIL fig. 5 ; mentioned also by GEOFFROY ST HILAIRE, Anom. d' Organ., i. p. 660.

197. A. vellerosus : extra molar on left side in lower jaw behind m?, as a fully-formed and well-shaped tooth, but not so large as m3. *199. Brachyteles hemidactylus [a genus doubtfully distinct from Ateles] : specimen from S. America having 1. upper series and all lower series normal. In place of right upper p1 are two teeth (Fig. 37). These two teeth are similar to each other and for want of space they bulge a little out of the arcade. Each is in size and shape very like normal p^, having a sharp cusp and a flat internal part to the crown. Both teeth are slightly rotated in opposite directions, so that the cusp of the anterior is antero-

lau-r:d in-t.-ail of lateral, while the cusp of the posterior is latrral. Tln'M- two teeth stand thus in somewhat complementary positions. 15. M., 42, a. *200. Ateles marginatus : wild specimen from river Cupai, has that is to say, an extra premolar on each side in the upper jaw, the lower jaw being normal. The four upper premolars are perfectly formed, large teeth, in regular series on both sides. As a consequence the lower canines bite on and partly beJiind the upper canines. There was nothing to suggest that any one of these teeth was supernumerary, rather than another (Fig. 38). B. M., 1.214, b.

201. Ateles ater : specimen from Peruvian Amazon : in right upper jaw there is a large alveolus for i-, which is gone, while a third incisor stands between this and the canine. This third incisor bites on lower canine, and lower pl of the same side bites in front of the upper canine. B. M., 1108, d. Absence of molars (cp. No. 209). Inasmuch as^jf, m\ is the normal formula for the Hapalidae, the following cases of absence of m3 in Ateles are interesting. There was in no case any doubt that the skulls wrere fully adult, and there was no suggestion that the absent tooth had been lost.

*203. Ateles marginatus : specimen from the Zoological Society's menagerie, bones rough and unhealthy-looking, but skull well formed and certainly not very young, has no ms in either jaw, giving the formula p — - , m-= — - , as in Hapalidae. There is no space in the jaw behind in-, and in the upper jaw the bone ends there almost abruptly. 204. A. melanochir : Caraccas specimen, having no posterior in3 on either side in upper jaw. The lower series normal, but the jaws are somewhat asymmetrical, so that the lower posterior right ms is behind the level of its fellow of the other side. B. M., 48.

205. A. variegatus : wild specimen, having lower m3 absent on both sides. Left ^ is also absent, but has been almost certainly present. C. M., 1098, B. Mycetes : of various species, adult normals, 81. Supernumerary molar. 206. M. niger : supernumerary molar in the right upper jaw. The arrangement is peculiar. So far as m- the teeth are normal. Behind and in series with m- there is a large tooth, a good deal larger than the normal m3, and having rather the form of inthan of m3. Its form is, however, not precisely that of m", for the middle or fifth cusp is rather anterior to the centre of the tooth,

in-t-ad . .f l.i-iii^ posterior to it as u.sual. Outside this tooth i> iii-tli- i -tan. ling out of the arcade, having the size and almost the will 1,.- in The next chapter pointed out as operating in the case • •I dfiiial \'ariati<in. Either in' may have divided into two, both -landing in series, and the normal m3 may have been pushed out • «f tlhaivad.- iii I'nimexion with this reduplication; or the tooth -•amling "in>id>' may represent an addition to the normal series, and in that <-a<>- the tooth standing as //<:i in the series may be a I* I'p sentation «\' m\ raised to the normally higher condition of m- in correlation \\iih th<- presence of an extra tooth in the series, in rln- \\ay -ln-\\n to occur in other cases (see Chapter x., Section 7). ll.-t \\i-i-n tin — ' ali«-niativc JM i>sibilities I cannot decide.

Mycetcs niger : In-tv.i-ui an.l iuti rnal to /^ and i^ on left side there is a .M>l;u . Tin> i- 1'inl.ahiv ;i -ui'i niuini i:iry «an', but the jaw is so much diseased Callithrix. iiMinial adult-. •_"_'. iln II. M., :, 1 , I, on both sides m? is separated narrow dia-t« ma iViijii "'-. The appearances sug^st that possibly % small rudimentary t.>,,th may IUIM- >tuod lutwci-u thum, but this is quite uncertain). aboul to hr formed, for tindentition is otherwise complete. C. M.. Cp. No. I'M-.'.)

Lagothrix, 6, Chiropotes, 1. Ouakaria, 3 normals respectively. II \i'\i. H' r.. In this 1,'nnip „/•' is normally absent: and no simen lia\im: tlii- t..otli or any other dental abnormality was seen. The evidence of the Variation of teeth in Canid;i' is divided into three groups according as it concerns (1) incisors, (2) prernolars, (3) molars. No case specially relating to the canines is known. In each of these groups the cases relating to (A) wild Canida? are taken first, and those relating to (B) domestic Dogs afterwards.

Of wild specimens of the genus Canis (including the Fox) 289 skulls were seen, and amongst them were 11 cases of supernumerary teeth, about 3'5 per cent, (besides many recorded cases). Of 216 domestic Dogs (including Pariahs, Esquimaux, &c.) 10 had supernumerary teeth, or 7'4 per cent, (besides many recorded cases). I have not included skulls of edentulous breeds, in which the original condition of the teeth cannot be told with certainty. Statistics of the occurrence of supernumerary teeth are given by HEXSKL, Morpli. Jahrb., 1879. Among 345 domestic Dogs in his collection there are 28 cases of one or more extra molars, 12 cases of extra premolar, and 5 cases of extra incisor. [If therefore no two of these cases refer to the same skull, there were in all 45 cases of extra teeth in 345 skulls, or 13 per cent. It is not stated that the collection was not strictly promiscuous, but it may be anticipated that this figure is rather high.] An analysis of Hensel's cases will be given in the sections relating to the particular teeth.

The usual dentition of the genus Canis is t|, c\, p^, m'^. The Wild Dog of Sumatra, Java and India, C. javanicus and C. primcevus (by some considered as one species) have wf and have been set apart as a genus under the name Oaon (HODGSON, Calcutta Jour. N.H., 1842, ii. p. 205). The genus Icticyon differs in having normally w|. The genus Otocyon on the contrary has usually m%. Of the variations to be described in Canis the most notable are (1) cases of iizi ; (2) cases of extra premolar, common in upper, very rare in lower jaws ; (3) cases of ^3 or ~m±, and one case of m% giving the formula characteristic of Otocyon. In several instances a considerable increase in the size of '»^ or m? is found associated with the presence of ^ or m* respectively. An interesting group of cases of extra molars was found in C. cancrivorus, in which this abnormality seems to be common.

The frequent absence of pl in the Esquimaux dogs is worth notice. Absence of ^ is common in Dogs, but absence of m?_ is rare. In Otocyon one case of m± is recorded, and in Icticyon one example has wif instead of m^. No ' ise of extra incisor known t<- me. I 'wo cases of absent incisor, viz. '0 : apparently f1 has not been present on either side. B. M., 212 Dog i r. -milling Bloodhound) : four incisors on each side in upper jaw. Tli.- .-\ternals, i3, normal, but no evidence as to which of ih' "tluT teeth supernumerary. Leyden Mus.

2lo. Thibetan Mastiff, Nepal: sockets for four teeth on each side in pmx. Teeth ,-,11 Li'"iie. Alveoli of two sides nearly symmetrical. In absence • •»' the t.-,-tli it cannot be positively stated that this is not a case ..I persistent milk t.-.-tk but this seemed unlikely. L>. M., 166, //. -I I- Mastiff: fmir ti-t-th on eacli side in front of canines; from form of teeth probably case of persistent milk-canines. Lower jaw gone. O. .M.. 217. Ann.ii'j :U"> Dogs' skulls tour had extra upper incisor on one -ide, and om- >knl| ha.l p.-rt'.-ct ly tormeil fourth upper incisor "ii li'it/i <ide>. 'I'liis lootli smaller than third incisor. HEXSEL,

Bulldog: li-lit | with very wide crown ; main cusp partially bitid as if Intermediate betueen single and double condition, Morph. /.nl>. ( Cambridge. 220. Absence of incisor is very rare in Dog. One case of i- given by HEXSEL, I c. p. 534. (Hensel observes that this gives the formula for incisors of Enliydris [Lata.f] ; he also calls attention to rig. of Enhydris with three lower iurixn,-* in O\VEN, Odontoyr., PL 128, fig. 12, but as this is not mentioned as an anomaly in text, it is very doubtful.)

Dog having the upper canine imperfectly divided into two on each side as shewn in Fig. 39. The plane of division was at right FIG. 39. Right and left profiles of Dog having the canines partially divided. angles to the line of the alveolus so that the two parts of each canine stood in the plane of the series of teeth. The division was more complete on the right side than on the left. The lower canines were normal. This specimen was kindly sent to me by Mr J. Harrison.

Several distinct variations were found in the premolars of Gamete. A number of cases shew five upper premolars instead of four, and the question then arises whether the extra tooth is due to the division of a single tooth, or to reconstitution of the series1. The occurrence of a fifth premolar in the lower jaw is much rarer, only three or four cases (Wolf (2) and Greyhound (? 2)) being known to me. The following other forms of Variation occurred. In C. mesomelas, No. 228, an extra tooth stood internal to pj, and was perhaps a duplicate of this tooth. One case of bifid X was seen, and two cases in which p2 had apparently divided to form two single-rooted teeth (C. viverrinus, No. 227 and a Sledge-dog, No. 237). A few examples of absence of pl deserve notice. Lastly, though really an example of Substantive Variation, I have included a curious case of possibly Homoeotic variation of p^ into the partial likeness of the carnassial (No. 245).

C. dingo : specimen having two closely similar teeth between j>- li.' ..mine in each upper jaw1. Both the teeth had the form and size of ' ]'r.-mi>lar. This not a case of persistent milk-tooth, NEHKINC. C. dingo : on right side /^ is in place, and there is an alveolus for • ml tooth of about same size. On left side p^ is rather small. L. M. C. viverrinus : left /•'-' re]ne-. nte.l by t\v.» teeth, each having one I'm it. Of the-e the anterior is tubercular, while the posterioiis rather IOULT from lirf'ore b;iek\v;mls. Anterior premolars normal. L. M. (Compare Sledge-dog, No. •_'."> 7.)

230. C. VUlpes : riglit i>\ has three roots and a partially double crown with two cusps (Fig. 41). The whole crown is pyramidal, the labial face being parallel to the arcade and the three roots stand each at one angle of the base : left 1^ normal; lower jaw missing. B. M., 175, o. FIG. 41. Teeth of Fox (C. rulpes) described in No. 230. The separate view shews the right first premolar removed, seen from the labial side. 231. C. lupus : two teeth between p* and canine in lower jaw on ri<///f side, one case : and the same on left side also, one case. These two occurred in 27 Wolf skulls seen by HEXSEL, Morph. Jahrb., 1879, v. p. 548.

*232. Dog : between p^_ and canine on rt. side there are two teeth, each shaped like a normal £\ the anterior being somewhat the larger. This seen in two cases, figured in Fig. 42, II. and III. FIG. 42. Profiles of canines and anterior premolars in three dogs having two teeth on one side in symmetry with one tooth on the other. in place, of proper form and size, standing evenly without crowding. •23 j. Dogs. h» 345 skulls were 1 1 cases of supernumerary premolar in tinuj'i'f r jaw, viz.

These were all cases described by HEXSEL as instances of the presence ..f ••;/" of his notation, i.e. a tooth between pl and canine. HEXSEL, J/../7>/i. Jahrb., 1879, v. p. 546. Out of 650 skulls, including Hansel's :'. l.'i, IS had two anterior premolars as described, on both sides in upper jaw. XKHUIXG, Xitzb. natnrf. Fr. JJerL, 1^82, p. 66. English Spaniel : oittxidf and anterior to right P_l is a worn stump, probably of au extra tooth (?). B. M., 166, j.

•J.'il!. Deerhound : tiro alveoli where ,7i should be; probably two distinct teeth stood h.'iv. inn it i< i">--il>le that the two alveoli were for distinct roots of a single tooth. -:>~ Sledge-dog, ( Ireenlaud : all teeth normal except left upper //-'. This tooth normally of course has two roots. Here it is r.-pr.-.-iit.-d by two distinct teeth, each having one root. The anterior ha- a fairly sharp cusp, but the posterior has a rounded emu ii. Tinteeth are in perfectly good condition and do not look worn. The\- aiv separated from each other by a considerable diastema. It apiH'ar- d^aithat insti-ad «>f the normal p*, two distim-t ircth have ln-cii t'ui'ined. 0. M., 1787 (compare C. viverrinus, No. i'^7 .

The following cases of absent premolars were doubtful : C. dingo : ri^ht P^ and left yT1- C. S. M. C. antarcticus: pl above and below on left side. C. S. M., tl.'i".. 242. From the nature of the case it is not often possible to say with confidence that pl has not been present in a given skull, but from the material examined this variation appears to be rather rare. In 216 skulls, excepting those of Esquimaux dogs, I only saw two clear cases in which the bones were smooth, without trace of alveolus, viz. "Danish" Dog: ]Tl absent on both sides, O. M., 1786. Terrier: y absent on both sides. C. S. M., 579. Many others doubtful.

According to HEXSEL, however, absence of />' is common, and he states that in 345 skulls the following occurred : Morpk. Jahrb., 1879, p. 546. [This is of course a far higher frequency than was found by me, but perhaps discrepancy arises from difference in reckoning the evidence of absence.] *243. Esquimaux Dogs : absence of/)1 quite common, the following skulls being all of the breed that I have seen. The partial establishment of a character of this kind in a breed, which, if selected at all, has been selected for very different qualities, is rather interesting. It need scarcely be remarked that the partial loss of this tooth cannot in the Esquimaux dog have occurred in connexion with an enfeebled habit of life, as might perhaps be supposed by some in the case of the edentulous lap-dogs.