SCIUKOMORPHA. 833 



The above measurements are taken from the largest specimens. In 

 some of the smaller ones the length of the anterior three inferior molars is 

 .0085, and in one instance .0080. I think that it was on a specimen of this 

 species with the last-named measurement that Professor 0. C. Marsh estab- 

 lished his Allomys nitens. The species was very briefly described, but may 

 be recognized by an accompanying figure. A new generic name was pro- 

 posed, but no characters were given, and it was even made the type of a 

 new family, but no reasons for this course were adduced. 



ISCHYROMYS Leidy. 



Proceed. Acad. Nat. Sci. Philad., 1856, p. 89 ; Extinct Fauna Dakota and Nebraska, 335. — Colotaxis Cope, 



Paleontolofjical Bulletin, No. 15, p. 1. 



10 5 



Ohar. gen. — The essential features are, dentition, I., ; C, - ; M., - ; 



the molars with two crescents on the inner side above, each of which gives 

 rise to a cross-ridge to the outer margin. In the mandibular series the crests 

 and crescents are identical, but in a reversed relation. No cementum. 



To the above characters given by Dr. Leidy, I have added the absence 

 of postfrontal processes, and the superior position of the infraorbital fora- 

 men; also that the pterygoid fossa is large, and that its inner and outer 

 plates are well developed, and subequal. The palate is excavated posteri- 

 orly. The acuminate anterior part of the malar bone extends as far for- 

 wards as the front of the orbit. There is no tuberosity on the side of the 

 superior diastema near the premolar teeth. In the mandible the posterior 

 extremity of the incisive alveolus is not distinct from the ascending ramus. 



In Hayden's Bulletin, vol. vi, I described some of the characters of the 

 remaining portions of the skeleton. Those which I possess are the distal 

 extremities of humeri, and a tibia, astragali, and portions of the pelvis. 

 The condyles of the humerus are narrow anteroposteriorly. The internal 

 flange descends at once to the fundus, leaving a long external cylindrical 

 roller without intertrochlear ridge. Anteriorly this portion is cut into for 

 half its length by the ligamentous fossae. There is a large internal epicon- 

 dyle, which is constricted by a neck at the base, and presents a compressed 

 edge inwards and upwards. The arterial canal opens above on the interior 

 side of the humerus. There is no external epicondyle. 

 53 c 



