| OCR Text |
Show 1896.] MAMMALIAN DENTITION. 579 correct, in which case the above objection would not hold, and the non-replaced pm. 1 may be regarded in all cases as a persistent milk-tooth. The presence or absence of the 1st premolar appears to be intimately connected with the development of the canine, for in mammals, other than the Insectivora, it is commonly wanting or much reduced in all those forms possessed of a large canine tooth, while in those forms in wdiich it is present in both dentitions the canine is either vestigial (Hyrax) or separated from the premolars by a wide diastema (Tapirus indicus). In the case with no succession to pm. 1, I should imagine that enlarged deciduous canine caused a slight decrease in size of dpm. 1, while the enormous permanent canine, which always develops early, caused a total suppression of ppm. 1 ; on the other hand, in those cases where pm. 1 is replaced, the non-development of the canines or their early removal forward allows the germ of ppm. 1 to mature and become functional. In forms such as the Pecora, in which both the canine and pm. 1 are wanting, this latter tooth was probably suppressed in some ancestor in w h o m the canine was well developed, and probably all trace of its germ has been lost, so that the subsequent loss of the canine has not caused pm. 1 to reappear; besides in these forms, as also in Equus, the posterior premolars have been so much enlarged that the anterior cheek-teeth became functionless and aborted. Osborn (32) on palaeontological evidence regards the single pm. 1 as a persistent milk-tooth. The Molar Teeth of the Mole. The lingual development of the dental lamina in relation to --^ is most conspicuous, it being more strongly developed in the Mole and Centetes than in any other animals I have examined, so much so that it is highly suggestive of a rudiment of a successional tooth (Plate X X V I . fig. 32, dl.); a similar but slighter growth is found in relation to m. 2. The Cusps. The molar teeth belong to the trituberculo-sectorial order; in lower molars the heel is very large and bears two strong cusps; the heel in niTl is larger than the trigon, but in m. 2 and m. 3 it is smaller; in all the protoconid is the largest and the paraconid the smallest of the main cusps ; a small posterior cingulum-cusp is seen in m. l, while m. 2 bears in addition a similar anterior cusp, in m. 3 the anterior one alone is present. The upper molars (Plate X X V I. fig. 35) are mainly tritubercular, but a very small hypocone (s) is present; the protocone (7) is small, whereas the paracone and metacone (5 & 6), especially the latter, are very large and show a tendency to become crescentic or V-shaped, the summit of the cone being situated some distance from the outer border of the |
