54 AMERICAN MESOZOIC MAMMALIA 



matrix impression of this part is known, there is little chance of error. The space is 

 large enough to hold one tooth, whether premolar or molar. Next follow two well 

 /' preserved molars. Posterior to these are alveoli for one more molar, then a space where 

 the inner part of the jaw is broken revealing a cavity which may have lodged another, 

 unerupted molar, although this is purely hypothetical. There were thus not less than 

 six nor more than seven cheek teeth, of which probably two and certainly not over 

 three were premolars. The formula may have agreed with that of Paurodon or of 

 Archaeotrigon distagmus. 



On the preserved molars, the paraconid points forward and upward and is as 

 large as in Dryolestes. The metaconid is directly posterior to the pa"* and is only a little 

 less high than the pr''. The latter is sharply pointed but has a very broad base. The 

 talonid is more of dryolestid than of paurodontid type, being low, with a distinct cusp 

 directly posterior to the me''. The trigonid, however, is longer than broad, the molars 

 are supported by two equal roots, and the metaconid, while high for a paurodontid, 

 would be low for a dryolestid. 



Mandible 



The horizontal ramus is markedly more slender than in the two preceding genera 

 and the symphysis is relatively longer. The internal groove is well marked and is 

 parallel to the lower border where preserved. 



Measurements 



Length M,2 1-35 mm. 



Length M^g 1.5 mm. 



Depth of ramus below M,, 2.5 mm. 



DRYOLESTIDAE Marsh 1879 



Definition. — Cheek teeth P4 M7 or more. Lower molars with four cusps. Meta- 

 conid comparable to protoconid in height. Trigonid of middle molars shorter than 

 broad, compressed anteroposteriorly. Talonid shorter than in Amfhitherium, triangu- 

 lar in plan, with a single posterointernal cusp. Molars implanted by two very unequal 

 roots, the larger external and anterointernal, the smaller posterointernal only. Lower 

 jaw long and slender. 



Type. — Dryolestes Marsh. 



Distribution. — Purbeck beds, England. Morrison formation, Wyoming and 

 Colorado. 



The various English and American genera referred to this family are very closely 

 similar, in so far as they are based on lower jaws. Once the molar structure is com- 

 pletely known, indeed, the separation of the genera becomes much more difficult than 

 was previously supposed and rests almost entirely on slight differences in the shapes, 

 proportions, and placing of the cusps. With this strong resemblance before one, it is 

 remarkable to find that they have almost never previously been placed in a single 

 family. In 1879 Marsh proposed two families, the Dryolestidae and the Stylodontidae, 



