78 AMERICAN MESOZOIC MAMMALIA 



Y.P.M. No. 10656B. Right maxilla with M'*. Fig'd (as Dryolestes), Osborn 

 1904, PI. XXI, fig. IB; 1907, fig. 14B. 



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



In defining Dryolestes obtusus, Professor Marsh gave as distinguishing features 

 the blunt cusps, short crowns, and projection of the last premolar beyond the rest. The 

 first two features are entirely due to wear and corrosion, and the last must have been a 

 misapprehension, for it is not now shown by the specimen. The jaw cannot be identified 

 either generally or specifically and Dryolestes obtusus is a nomen nudum.^'^ 



Osborn (1904) later studied both the specimens here under consideration. Unfor- 

 tunately, at the time of his visit to New Haven the larger part of the material of upper 

 jaws now described for the first time was set aside and had not been cleaned or cata- 

 logued. He was consequently obliged to confine himself to this material which is so 

 badly preserved as to be almost wholly uncharacteristic. With the exception of some 

 details which could be visible only to an observer who had access to unworn compara- 

 tive material. Professor Osborn's figures of these two jaws are very accurate. This lack 

 of comparative material and the impossibility of then appreciating the inadequacy of 

 these specimens, however, inevitably led to some erroneous impressions despite the 

 accuracy of observation. Some of these errors have been adopted into the literature 

 (e.g., by Gregory) and they must be briefly corrected : 



1 . No known dryolestid upper molars have an internal cingulum. The line here 

 so called is simply the limit to which corrosion has removed the enamel. 



2. There are three, not two, external cusps and the highest is central, not anterior. 



3. The "external cingulum" noted by Osborn is analogous to the internal one, 

 being the limit of corrosion. There is a sort of cingulum in these upper molars but it is 

 only a ridge running backward from the tip of the anteroexternal cusp for a short dis- 

 tance and it has been entirely removed by wear and corrosion in the present instance. 



4. The largest external cusp is not united to the internal cusp by a median ridge 

 in Herfetairus but it is in Melaiwdon. It is impossible to say which was the case here. 



5. The posterior, not the anterior, ridge is normally stronger and it bears a well 

 marked cusp when unworn. 



An inadvertent error in the figure (Osborn 1904, PI. XXI, fig. ia) must also be 

 noted. In the palatal view the teeth are correctly labeled, but in the external view "P*" 

 should be P^ ; "MS" P* ; "MS" M^ etc. 



The small foramen labeled infraorbital foramen by Osborn is only one of two 

 outlets of the infraorbital canal. The main infraorbital foramen was much larger and 

 was located at the anterior edge of the specimen, directly anterior to this smaller 

 branch, and above the anterior root of P*. 



The structure of the other specimen available to Osborn is even more obscure. The 



^^ There is some slight doubt as to whether Y.P.M. No. 11819a or 10656b is the type of this 

 species. As Marsh left the collection both were together in a small box labeled "Dryolestes obtusus 

 (type)." The former is the better specimen, however, and a later manuscript list of types by Marsh 

 seems to put the matter beyond reasonable question. In either event the name is a nomen nudum. 



