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Show 1896.] MAMMALIAN DENTITION. 569 with the fourth tooth, the so-called 14, but here the labial enamel-organ is more marked and bell-shaped (Plate X X V . fig. 18, dc). The gap between the premaxilla and maxilla at this stage is so extensive that the germs of several teeth are contained within it, it being quite impossible to identify the future boundary line between maxillary and premaxillary teeth (fig. 19). But even if it be a fact, as Brandt states (1 a), that the four anterior upper teeth are situated within the confines of the premaxilla1, I fail to see that it is proven that these teeth are the homologues of the four incisors seen in the Polyprotodont Marsupials, but would rather be inclined to regard Brandt's fourth incisor as a canine, abnormally situated ; for among the Insectivora this tooth is very variable in its relations to the premaxillo-maxillary suture, due probably to the variations in relative extension of these two bones, the canine itself remaining constant in its position. The 2nd and 3rd lower incisors were in a very backward condition ; both, however, exhibit labial growths of the dental lamina, that connected with [72 being the most marked (fig. 20). The 3rd lower incisor is, however, a vanishing structure and does not develop into a functional tooth (fig. 21). In the upper jaw the tooth which I regard as the anterior premolar, usually called the canine (Brandt), was difficult of identification, it being hardly differentiated from the dental lamina (fig. 19, pm. 2?). Of the two undoubted premolars the posterior is the largest and the most advanced in development, the enamel-germ of the anterior tooth being still in the club-shaped stage, but possessing a well-marked cup-shaped labial (fig. 22) enamel-organ belonging to its vestigial predecessor. A similar structure to this, but more highly differentiated and of still larger size, is attached to the posterior premolar (fig. 23, dpm. 4), which, from its large size and close proximity to the molar teeth, is probably the true 4th premolar (ppm. 4). These three teeth I regard as premolars; in all probability they represent pm. 2, 3, & 4. The enamel-organ of the single lower premolar, like ppm. 4, was large and highly differentiated (fig. 24) ; it also exhibits the labial enamel-organ of its vestigial predecessor. The Molar Teeth. In the foetus examined by me m' 1'm'" were distinguishable, but rf m. 1, m. 2 b ' not very advanced in their development; -^- exhibited slight lingual continuations of the dental lamina. 1 A reference to Brandt's figures will show that it is only in Crossopus (fig- 2) that the 4 anterior teeth are quite within the limits of the premaxilla; in Sorex (fig. 1) the premaxillo-maxillary suture is so represented that the fourth tooth is situated in the gap between the two bones, a condition characteristic of the 4th tooth or canine of m a n y Placentals. PEOO. ZOOL. Soc-1896, No. XXXVII. 37 |
