MORRISON MAMMALS: SYMMETRODONTA 



41 



Amp/ilJon Simpson 1925 

 1925. Ampfiidofi, Simpson, Atm-r. Jour. Set. (5) X, 460. 



Definition. — Sole known genus certainly referable to the family. Dental for- 

 mula I, Ci P4 M4. 



Tyi'e. — A. suferstes Simpson. 



Distribution. — Morrison formation, Wyoming. 



Amph'idon super stes Simpson 1925 

 1925. A. superstes, Simpson, Amer. Jour. Set. (5) X, 460. 



Xype. — Y.P.M. No. 13638. Part of right lower jaw, with P4 and M1-4. 

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

 Diagnosis. — Sole known species of the genus. Lengths of molars i to 4, 1.3, 1.2, 

 I . I , and 0.7 mm. respectively. 



Incertae sedis 



Etirylatnbda, new genus 



Definition. — Upper molars triangular, about twice as long as wide, with high 

 centrointernal main cusp. Antero- and posterointernal shearing edges, the former with 

 one and the latter with two small cuspules. Mo- 

 lar somewhat basined externally, with a raised 

 external cingulum. No internal cingulum. 



Type. — Eurylambda {" Amp/tidon") ae- 

 qu'icrurius (Simpson). 



Distribution. — Morrison formation, Wyo- 

 ming. 



This is the only upper molar type in the 

 American Mesozoic referable with any probabil- 

 ity to the Symmetrodonta and it also represents 

 a t\-pe of upper molar new among the Mammalia. 

 It is therefore essential that it should bear a 

 distinctive name. In 1925 the writer referred it 

 provisionally to Ampliidon, although noting 

 that it would require a somewhat shorter lower 

 molar than those of the genotype and suggesting 

 possible relationship to Tinodon. (Simpson, 

 1925c, p. 464.) At the same time the resem- 

 blance to the English Peralestes was emphasized 

 and it was considered possible that Peralestes 

 represented the upper dentition of an otherwise 

 unknown amphidontid. Since that date, how- 

 ever, the originals of Peralestes have been stud- 

 ied and it has been found that the latter is with- 



(^ 



h 



^ ^ A ^% 



' ti m ^ 



Fig. 18. Morrison mammals. Internal, 

 crown, and external views of typical 

 right lower molars. A, Priacodon. B, 

 Atnphidon. C, Tinodon. D, Paurodon. 

 E, Dryolestes. F, Docodon. Not to 

 scale. 



