822 THE WHITE EIVER FAUNA. 



groove is marked on the inner side of the distal portion of the shaft, its 

 anterior border being especially well defined 



Two species of this genus are certainly known. They belong to the 

 White River horizon of Colorado. They differ, so far as known, chiefly 

 in size, and in the proportions of the inferior premolar tooth. 



Gymnoptychus minutus Cope. 



Paleoatological BuUetin No. 16, p. 6 (August 20, 1873). Annual Report U. S. Geol. Surv. Terra., 1873 



(1874), p. 476. 

 Plate LXV ; figs. 19-30. 



This species is about the size of the Dipodomys phUipsi, and was abun- 

 dant during the White River epoch in Western North America. The speci- 

 mens which I obtained in Colorado represent twenty or more individuals. 

 Among them are portions of four crania. One of these presents characters 

 somewhat different from those seen in the best preserved of the others, and 

 may belong to another species of the genus. To this supposed species I 

 formerly gave the name of Gymnoptychus nasutus. 



The Gymnoptychus minutus was originally described from mandibular 

 rami. The cranial fragment which agrees with these in the size of its teeth 

 is the portion anterior to the middle of the orbit, and lacks the greater part 

 of the nasal bones. The maxillo-premaxillary suture is quite sinuous in its 

 inferior portions, and passes in front of the infraorbital foramen, measuring 

 one-fourth the distance from the latter to the inferior border of the incisive 

 alveolus, or about two-fifths the distance from the last-named point to the 

 first molar tooth. The incisive foramina extend chiefly in advance of this 

 suture, notching the maxillary bone a little. A shallow groove entends 

 posteriorly from each incisive foramen along the palate as far as the pala- 

 tine foramina, becoming shallower posteriorly. The tuberosity behind the 

 inferior extremity of the infraorbital foramen, usual in the Sciuridce, is here 

 represented by a scar, with an inferior angular border. A shallow and wide 

 fossa occupies the entire side of the maxillary bone above the infraorbital 

 foramen and behind the premaxillary suture. The side of the premaxillary 

 anterior to the suture is also shallowly concave. The maxillary is also 

 slightly concave in front of the inferior base of the zyomatic arch The 

 base of the latter has considerable vertical extent, but the arch is not 



