MORRISON MAMMALS: PANTOTHERIA 59 



Neotype. — U.S.N.M. No. 2722. Right lower jaw with P2.4 and M1-7." 



Types of Synonyms.— Siy/acodon validus: Y.P.M. No. 1 1884. Right lower jaw 

 with Pa-., and M1-4. Asthcuodon segnis: Y.P.M. No. 10646. Right lower jaw in matrix, 

 crushed, external aspect, with P1.4 and M,-7. Fig'd Marsh 1887, PI. IX, fig. 7. 



Horizon and Locality. — Morrison formation, Como Bluff, Wyoming. The type 

 was found at the north end of the bluff, just under the sandstone capping, hence at a 

 higher level than any other mammals from this locality. The other specimens are from 

 Quarry 9, with a single exception. 



Diagnosis. — Sole species of the genus. Dimensions as below. 



This was the first Mesozoic mammal found. Like most harbingers of new fields, 

 the type is very imperfect. The single tooth preserved is badly worn and lacks the main 

 cusp. After prolonged study and careful comparison, it has seemed possible to desig- 

 nate a proper neotype with sufficient probability, thus saving this historic and widely 

 used name. Marsh's figured specimen (1887, PI. IX, fig. 2), Y.P.M. No. 137 19. can- 

 not be accepted as the neotype. It was not so designated by Marsh, and it seems improb- 

 able that it is congeneric with the type. Certainly it is not congeneric with the specimen 

 here made neotj^e. 



Stylacodon validus was described when neither Stylacodon nor Dryolestes were 

 well known. It would have been withdrawn from the former genus had Marsh ever 

 revised the group, for it shows none of the characters given by him as characteristic of 

 Stylacodon, while, as he said, the molars agree with those of Dryolestes. It shows no 

 specific differences from the neotype. 



When the supposed generic characters of Asthenodon are recognized as erroneous, 

 there remain no specific distinctions between Asthenodon segnis and the neotype of 

 Dryolestes -priscus. 



Dentition 



The crowns of the incisors and canine are not known in this genus, but they were 

 probably much like those of the related Laolestes. The premolars, four in number, are 

 of identical pattern, but differ in size and proportions. Pi and P- are of about the same 

 size. P3 is somewhat larger and higher, about equaling Mi in height, while P4 is nota- 

 bly the largest tooth of the premolar series, overtopping the first molar. The external 

 surface of each tooth is convex. There is a small anterior accessory cusp near the tip of 

 the high somewhat recurved main cusp. From this accessory cusp there runs down on 

 the internal surface a sharp vertical crest separated from the base of the main cusp by 

 a short vertical valley. The crest curves somewhat posteriorly at its lower end but dis- 

 appears without crossing the base of the main cusp. Back of the main cusp at a low level 

 is a small posterior cusp or heel. Its inner surface is also sharply separated from that of 

 the main cusp and the internal ridge running downward from it curves forward and 

 becomes horizontal inferiorly before disappearing. 



" Through inadvertence, Y.P.M. No. 10646 (type of Asthenodon segnis) was given as the neotype 

 in Amer. Jour. Set. (5) XIII, 411. 



