1896.] MAMMALIAN DENTITION. 569 



with the fourth tooth, tlie so-called i^, but here the labial enamel- 

 organ is more marked and bell-shaped (^Plate XXV. fig. 18, dc). The 

 gap between the preuiaxilla 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 premaxilla', I fail to see that it 

 is proven that these teeth are the hoinologues 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 con- 

 dition; 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 identi- 

 fication, it being hardly difierentiated 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 pre- 

 molar (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 ppta. 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 *"' ,'™".^ were distinguishable, but 

 not very advanced iu their development ; 2^ exhibited slight 

 lingual continuations of the dental lamina. 



' A reference to Brandt's figures will ehow that it is only in Crossopua 

 (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 characterishic of 

 the 4th tooth or canine of many Placentals. 



Proo. Zool. Soc— 1896, No. XXXVII. 37 



