MORRISON MAMMALS: SYMMETRODONTA 



37 



Diagnosis. — Grooves of wear 

 on upper molars such as to pre- 

 vent occlusion with lower molars 

 of P. robustus. Lengths of first 

 three upper molars about 2.3, 3.0, 

 and 2.5 mm. respectively. Broad 

 palatal groove internal to raised 

 molar border. External molar 

 cingula continuous but faint. No 

 internal cingular cusps. Bases of 

 upper molars gibbous. Anterior 

 border of orbit above last pre- 

 molar. 



Priacodon grandaevus 

 Simpson 1925 



1925. P. grandaevus, Simpson, Amer. 

 Jour. Set. (5) X, 350. 



Fig. 14. Palate of Priacodon, seen from below. The cross- 

 lined portions are restored. Ch, internal nares; g, broad 

 groove internal to raised alveolar border (not seen in all 

 species of Priacodon) ; mx, palatal process of maxilla; pal, 

 palatal process of palatine; pf, palatal vacuity. (After 

 Simpson, 1925B.) 



Type. — Y.P.M. No. 10349. 

 Part of left maxilla with M^"'. 



Referred Specimen. — U.S.N.M. No. 2698. Part of right maxilla with M^"'. 



Horizon and Locality. — Morrison formation, Quarry 9, Como Bluff, Wyo- 

 ming. 



Diagnosis. — Grooves of wear on upper molars such as to prevent occlusion with 

 lower molars oiP. robustus. Lengths of first three upper molars about 2.1, 2.3, and 2.4 

 mm. respectively. Molar border little raised. No palatal groove. External upper molar 

 cingula sharp, continuous. Internal cingular cusps present. 



Order SYMMETRODONTA Simpson 



Definition. — Lower molars nearly or quite symmetrical with respect to a trans- 

 verse median plane, with a high centroexternal cusp, lesser anterior and posterior in- 

 ternal cusps, and a well developed internal cingulum below the latter. No true heel or 

 talonid. Upper molars with a high centrointernal cusp, anterior and posterior external 

 cusps on its slopes, and an external but no internal cingulum. Incisors not enlarged, 

 canines present. Mandible with slight pterygoid crest but no angular process. 



Distribution. — Upper Jurassic of England and the United States. 



One of the greatest recent advances in the interpretation of Jurassic mammals is 

 the perception, first by Matthew (in Gregory 1922, p. xiii), of the fact that the forms 

 which the writer calls symmetrodonts have no intimate relationship with the tricono- 

 donts. As already pointed out in dealing with the latter group, almost all classifica- 



