616 ON THE SKULL OP THE FOSSIL REPTILE DIADEMOD0N. [May 26, 



external crenulate border and a median crenulate transverse ridge, 

 dividing tlie concave posterior half of the crown, which has a crenulate 

 external ma,rgin, into larger external and smaller internal concave 

 spaces (see text-fig. 1 30). In front there is a sharp or crenulate 

 marginal border, with transverse crenulations or cusps ; on the 

 middle of the crown two small anterior cusps and two postei'ior 

 cusps. These crowns are essentially of the type of the described 

 species of Diaclemodon. The last tooth of D. mastacus shows a ten- 

 dency to develop a posterior talon {I. c. 1894, B, pi. 89. figs. 11, 12). 

 The penultimate tooth of this specimen has the posterior talon so 

 developed as to make the form of the crown almost triangular. 

 The crown is only a quarter of an inch wide, and slightly shorter 

 from front to back externally. The strong external anterior cusp 

 is broken, but a small external cusp rises from the talon. On the 

 inner border of the crown are two or three cusps or crenulations 

 like those similarly placed on the fifth and sixth molars. The last 

 molar is compressed from side to side, -f^^ inch long by j\j- inch 

 wide, broader in front than behind, with small tubei'cles back and 

 front. The small size of these teeth gives the molars the aspect 

 of exceptional divergence posteriorly. The transverse internal 

 measurement between the last pair of molar teeth is 1 j^^- inch ; 

 between the fifth pair it is f inch, and between the first pair of 

 molars about \^ inch. From front to back the crowns form a 

 convex curve. 



The dentition is imperfectly preserved, but not more than two 

 or three premolar teeth appear to be lost. From the resemblances 

 of the skull to allied types I infer that there was a toothless 

 diastema between the first premolar and the canine, where the 

 jaw contracted from side to side. I should expect four incisors 

 as in GompJiognaikus. The missing extremity of the snout would 

 be about ly^^ inch long ; the missing hinder part of the head was 

 about 2| inches long, giving the complete skull a length of 

 6| inches. The skull may be restored on the type of Gompho- 

 gnatlms (text-fig. 130). 



The most remarkalole feature of the dentition is the unworn 

 condition of the crowns of the teeth, also seen in other species of 

 the same genus. The transversely ovate forms of the molar 

 crowns acquire new interest from the teeth of ProcolopJion 

 having this form, with inner and outer cusps recalling the tooth 

 of Diaclemodon broioni. But while Procolojihon is typically 

 reptilian in its dental armature (Proc. Zool. Soc. 1905, vol. i. 

 p. 225), in this fossil the teeth suggest mammalia. The trans- 

 versely ovate form of the crown, with the slight cingulum, 

 approaches the condition in lemurs, but the molars are more 

 numerous and the other dental characters unlike. The diastema 

 occurs among mammals as various as marsupials, tapirs, rodents, 

 but is never associated with a transversely ovate molar, and full 

 sei-ies of incisor and strong canine teeth as in these fossil reptilian 

 types. Mammals of various groups have the molar teeth 

 progressively increasing and afterwards decreasing in size, as 



