1276 CLASS MAMMALIA. 



and metaconid ; calling the talon of the present type the hypoconid. 

 Corresponding terms ending in cone are applied to the upper molars 

 of the same types. 



It will be obvious that when a jaw is embedded in matrix with 

 the inner surface exposed we shall only see the cusps a and c and 

 the talon d ; while when the outer surface is visible only the large 

 cusp b will be observed. An example of the latter occurrence is 

 shown in fig. 11 46; and this circumstance has been the fruitful 

 source of error in regard to a number of the Mesozoic types under 

 consideration. 



In the type genus Amphitherium of the Stonesfield Slate, or 

 Lower Jurassic, the exact dental formula is unknown, but it may 

 perhaps have been the same as in the next genus. 1 In Amblo- 

 therium ( = Peraspalax, Phascolestes) of the Purbeck, or Upper 

 Jurassic, of Dorsetshire, the lower dental formula is /. 4, C. 1, 



Fig. 1 146. — Outer view of the left ramus of the mandible of Amblotherium gracile ', from the 

 Upper Jurassic of North America. Three times natural size, a, Canine ; c, Coronoid process ; 

 b, Condyle ; d, Mandible. (After Marsh.) 



Pm. 4, M. (7—8), or the same as in the American Jurassic Dryo- 

 lestes (fig. 1 144). Several species are known, in some of which 

 there were seven, and in others eight lower molars. The genus 

 Stylodon has been founded upon the outer side of mandibles of 

 Amblotherium, in which only the large posterior cusp (protoconid) 

 of the blade of the molars is visible, as in the lower jaw shown in 

 the accompanying woodcut, which has been made the type of the 

 genus Stylacodon, of the Upper Jurassic of North America. The 

 latter has eight lower molars, while the English form has only 

 seven. Achyrodon, of the English Purbeck, is closely allied to 

 Amblotherium, but differs in the form of the cusps of the molars. 

 Peramus (Zeptocladus) is a third Purbeck genus, with relatively 

 stouter lower molars, in which Professor Osborn gives the lower 

 dental formula as I. 3, C. 1, Pm. 6, M. 3. The North American 

 Dryolestes has relatively shorter lower molars (fig. 1144), the wood- 

 cut clearly showing the three cusps of the blade and the talon of 

 these teeth. Asthenodon and Laodon are other North American 



1 Professor Osborn regards it as /. ?, C. I, Pm. 5, M. 6. 



