1910.] The Triconodonta. 173 



from mandibular rami with teeth. Upper teeth are known only in Trirono- 

 don and Peralestes. 



Naturalness of the Group. 



The question whether the order as thus defined includes any extraneous 

 elements or is a truly natural assemblage seems to be answered as follows: 

 The oldest genera Amphilestes and PJiascoIotherium from the Stonesficld 

 Slate present the triconodont type of molar in a low stage, reminiscent in 

 some features of the Upper Triassic Microconodon (}). 164). There is a 

 strong internal cingulum which rises opposite the main middle cusp. In 

 Triconodon and- Priacodon of the Upper Jurassic the anterior and posterior 

 cusps have become subequal with the main middle cusp. The coronoid, 

 known in Triconodon and Phascol other ium, is very broad, the condyle 

 sessile and confluent with the inflected angular border. Aside from the 

 progressive development of the anterior and posterior cusps in the molars 

 the chief differences between the genera lie in the dental formulas : 

 Amphilestes: 1. 5, C.j, P.j, M.g (Goodrich). 



Triconodon: I?|, C.i, P.|, M.f^ (Osborn). ^ 



Phascolotherium: 1. 5, C.j, P.g, M.g (Osborn, Goodrich). 

 So far as the published evidence goes the ordinal association at least of 

 Amphilestes, Phascolotherium, Triconodon and Priacodon seems to be war- 

 ranted. 



To judge from figures and descriptions Menacodon Marsh is also a related 

 genus. Its dental formula is I.^, C.j, P. 3, M.j (Osborn). Its premolars 

 resemble those of Priacodon (? Triconodon) in certain significant details; 

 the internal basal cingulum is present and the central cusp of the molars is 

 larger and higher than the anterior cusps. But these are now somewhat 

 internal to the main central cusps so that in the last molar the three cusps 

 are arranged in a triangle with sharp pricking points, the whole resembling 

 the molar type seen in Spalac other ium tricuspidens Owen. The ramus in 

 Menacodon is slender (more so than in Amphilestes) and the angle seems to 

 have been inflected. Spalacotherium as described by Owen and by Osborn, 

 resembles Triconodon in the canines and incisors and Amphilestes in the 

 premolars. In the lower molars the paraconid and metaconid are internal 

 to the protoconid and spring outward much as in Menacodon. The basal 

 cingulum is very broad and shelf-like. The angle was inflected, the condyle 

 confluent with the angle, and the coronoid rose very abruptly but was not 

 so broad as in Tinodon. The lower molars of Spalacotherium agree with 

 those of other Triconodonts and contrast with those of Trituberculates in 

 many points as shown below: 



