22 PROCEEDINGS OF THE NATIONAL MUSEUM vou 86 



specific designation. A fourth specimen, U.S.N.M. no. 10920, con- 

 sisting of an incomplete left maxillary containing a few teeth, has 

 now turned up in the collections. This specimen was also collected 

 by A. C. Silberling in 1908, in Sec. 4, T. 5 N., K. 16 E., Sweetgrass 

 County, Mont. In size and in all other particulars, insofar as they 

 can be compared, this maxillary and teeth are in full accord with 

 those of Peltosaurus granul^sus^ with which they have been directly 

 contrasted. In view of their much earlier geological occurrence, I 

 am loath to assign them to the Oligocene species, as in all probability 

 more complete specimens would show their specific distinctness. For 

 the present, therefore, I shall continue to regard these specimens as 

 specifically undeterminable. 



B 



S? 



FiGCBB 6. — Right maxillary of f Xestops piercei, new species, type (U.S.N.M. no. 13807) : 

 A, Lateral view ; B, posterior view of tooth. Five times natural size. 



Genus XESTOPS Cope 



7 XESTOPS PIERCEI. new apecies 

 FlQUBES 6, 7 



Type. — U.S.N.M. no. 13807, consisting of both maxillae, anterior 

 ends of both dentaries with numerous upper and lower teeth, and 

 many dermal scutes. Collected by George B. Pierce, 1935. 



Locality. — About 6 miles north of Tuttle Ranch, Elk Creek, Big 

 Horn Basin, Big Horn County, Wyo. 



Horizon. — GraybuU formation, Wasatch, Eocene. 



DescHption. — All the bones comprising the type specimen were 

 found cemented together in a compact mass by the iron-stone cover- 

 ing so commonly found adhering to Wasatch fossils. It is quite evi- 

 dent that originally the entire skull had been present, as the maxillae 

 and dentaries are little disturbed from their normal relationships, the 

 whole top of the skull having been eroded away. 



The right maxillary is complete in length, but the left one lacks 

 a small portion of its anterior end. The right maxillary has the usual 

 triangular outline, being deeply emarginated toward the upper an- 

 terior end and thus forming much of the lower and posterior borders 

 of the external narial opening. The external surface of the bone is 

 smooth, with only slight indication of the former presence of osteo- 



