TEETH OF THE MESOZOIC MAMMALS 21 



respect to the incomplete separation of the fangs and the remarkable 

 variations in the number and size of the accessory molar cusps. In fact 

 the molars appear to be in what may lie called an experimental stage of 

 structure. The accessory cusps are sometimes large and distinct, as in 

 the third true molar ; sometimes minute as needle points, as in the 

 second molar. The incomplete separation of the fangs is a reptilian 

 character, which when correlated with the styloid prernolars and recurved 

 canine-incisor series, places Dromafheriiim very remote from any of the 

 known Mesozoic mammals. Microconodon, on the other hand, is of a 

 somewhat more recent type, the premolars have the trace of a low 

 posterior heel, and the molars have that regular tricuspicl division of the 

 crown which is first observed in the genus Ainpliilestes (Fig. 5) of the 

 English Lower Jurassic and characterizes a large number of the Jurassic 

 mammals. 



2. 

 A NEW CLASSIFICATION OF THE MESOZOIC MAMMALS. 



[In order to make the following section clearer we insert here a brief 

 classification of the Mesozoic (Lower and Upper Jurassic) mammals of 

 the orders Triconodonta and Pantotheria (Trituberculata). ED.] 



A. Infra-class Marsupialia. 



1. ORDER: TRICONODONTA OSBORN. 

 1. FAMILY : TRICONODONTID.E MARSH. 



Probably carnivorous pro-Marsupials. Molars with three stout crest 

 cusps, the anterior and posterior cusps derived from the crown, and 

 a .strong internal cingulum, rising into anterior, median and posterior 

 prominences. Opposition of upper and lower molars subtrenchant. 

 Postcanine teeth 7 to 11, usually 7. Angle of jaw inflected. Coronoid 

 broad, recurved. 



a. Subfamily Amphilestince Osborti. 



Lower Jurassic. Anterior and posterior cusps of molars much lower 

 than middle cusps, all three cusps being in the same fore-and-aft line. 

 Postcanine teeth 9-11. Angle distinct in outside view. Mandible rather 

 slender. 



Genus Amphilestes. Lower Jurassic (Stonesfield Slate), England. 

 Figs. 4 (No. 1), 5. 



