yz AMERICAN MESOZOIC MAMMALIA 



Besides the genotype, a second species, "iHerfetairus humilis, is doubtfully re- 

 ferred to the genus. 



Herpetairus arcuatus (Marsh 1879) 



1879. Dryolestes arcuatus, Marsh, Amer. Jour. Sci. (3) XVIII, 397. 

 1927. Herfetairus arcuatus, Simpson, Amer. Jour. Sci. (5) XIII, 413. 



Type. — Y.P.M. No. 11822. Right maxilla with M^"^ much worn. 

 Neotype. — Y.P.M. No. 13740. Left maxilla with M^"*, almost unworn. 

 Referred Specimens. — Y.P.M. No. 13739. Maxilla with P* and W'^. 

 Y.P.M. No. 13742. Fragment of maxilla with two molars, provisionally referred. 

 The following less important specimens are definitely referred to this genus and 

 provisionally to this species : 



Y.P.M. Nos. 13741, 13743, 13744, 13746, 13747- 

 U.S.N.M. Nos. 2724, 2762, 2800, 2818, 2845. 



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

 Diagnosis. — Total length of molar series about 7.5 mm. No transverse median 

 ridge on molars. 



Dentition 



Judging from alveoli only, there appear to have been four upper premolars. This 

 number of premolars, both upper and lower, was probably common to all dryolestids. 

 No upper jaw, of whatever genus, is known in which there are surely more than seven 

 upper molars and this number is known to have been definitive for the present genus 

 and some others. Since eight is the normal number of lower molars in the family the 

 heel of Mg must have been functionless, although it is known to have been retained in 

 Amblotherium, at least. 



P* is high, piercing, slightly compressed, conical, not definitely recurved. There 

 is a cingulum, both internal and external. It is weak across the middle of the tooth, but 

 becomes sharp and strong posteriorly, forming a small posteroexternal cuspule. As in 

 the dryolestid lower jaws, the change from premolars to molars is very abrupt. What- 

 ever their history, in this family the premolars and molars retain no definite traces of 

 a common ancestral morphological type nor do the premolars show any tendency 

 toward molarization. 



The crown of each molar is obliquely triangular with its anterior border about 

 vertical to a line joining the internal cusps and its posterior border running outward 

 and markedly backward from this line. The width of the teeth'' increases from M' to 

 M^ then decreases slightly to M^ while the length remains nearly constant to M* and 

 then decreases to M^ From M' to M^ the teeth thus become relatively narrower, with 

 more acute internal apices, and M""^ have about the proportions of M^ but are smaller. 

 The internal cusps are arranged in a nearly straight anteroposterior line, arched very 



" Throughout this work the term "width" applies to the transverse measurement, regardless of 

 whether it is the larger or smaller dimension. 



