MORRISON MAMMALS: TRICONODONTA 31 



here given, however, with some changes made possible by study of European material 

 and with such data as are necessary to maintain the continuity of this memoir. 



TRICONODONTIDAE Marsh 1887 



Definition. — (With the characters of the order.) 

 Type. — Triconvdon Owen. 

 Distribution. — As for the order. 



AMPHILESTINAE Osborn 1888 



Definition. — Premolars symmetrical and submolariform. Molars more than 

 four in number, with the anterior and posterior cusps small relative to the main cusp. 



Type. — Am-p/iilestes Owen. 



Distribution. — Stonesfield slate of England and (?) Morrison formation of 

 Wyoming. 



This primitive subfamily is represented in England by Am-phUestes and Phasco- 

 lotherium from the middle Jurassic Stonesfield slate. One of the most unexpected re- 

 sults of the present work was the discovery in the Marsh Collection in the United States 

 National Museum of two new genera which can only be placed here, although they are 

 of uppermost Jurassic age and contemporaneous with the more highly developed tri- 

 conodontines. These genera, which have been named Phascolodon and Afloconodon, 

 are very poorly known and their position in the system may possibly be changed by the 

 discovery of better material. 



Phascolodon ^'ivn^son 1925 

 1925. Phascolodon, Simpson, Avier. Jour. Set. (5) X, 334. 



Definition. — Molars probably more than four. Accessory cusps about half the 

 height of the main cusp, the latter rather blunt, and gibbous externally. Internal cin- 

 gulum curving upward slightly in the middle and rising to form small anterior and 

 posterior cingulum cusps at the ends. Last molar much reduced in size, similar to pre- 

 ceding tooth but without cingulum cusps. 



Type. — P. gidleyi Simpson. 



Distribution. — Morrison formation, Wyoming. 



The molars of this genus are strikingly like those of Phascolotherium, but it can 

 readily be distinguished by the smooth enamel, by the character of the cingulum, and 

 by the reduction of the last molar. 



Phascolodon gidleyi Simpson 1925 

 1925. P. gidleyi, Simpson, Amer. Jour. Set. (5) X, 334. 



Type. — U.S.N.M. No. 2703. Fragments of left lower jaw, with the last four 

 molars. 



