MYOMORPHA. 867 



The chai-cacters of the species are as follows: 



I. Otie and mastoid bullae continuous. 



Temporal ridges uniting into a sagittal crest; length of skull .043; supraorbital 

 ridges and concave front P- sulcifrons. 



Temporal ridges not uniting; lengtk of skull .035; interorbital region flat; no 

 ridges -P. leptophrys. 



II. Otic and mastoid bullie separated by a deep groove. 



Temporal ridges not united; front concave; size medium; supraoccipital 

 wide -P. diplophysus. 



Pleurolicus sulcifrons Cope. 



Paleontological Bulletin, No. 30, p. 4. Proceedings American Philosophical Society, 1878 (1879), p. 66. 



Plate LXIV; fig. 6. 



Five crania represent this species in my collection; only one of these 

 includes the posterior portions. In two of them the molar teeth are well 

 worn; in one (the type) they present a medium degree of wear, and in two 

 the roots only are preserved. This species resembles those of the allied 

 genus Entoptychus in many respects. The superciliary borders are thickened 

 upwards, forming two ridges, which inclose a groove between them which 

 is more pronounced than in most specimens of the Entoptychus cavifrom. 

 The muzzle is plane above and considerably wider than the interorbital 

 space. The base of the malar is thin and oblique, and the foramen infraor- 

 bitale exteritis is well in advance of the molar teeth and at the anterior part 

 of the maxillary bone. A groove passes backwards from its inferior border, 

 terminating in a small foramen which marks a point nearly half way to the 

 first molar. This foramen is present in all the crania. Within this another 

 shallow groove bounds the more prominent median line. The palatal sur- 

 face exhibits two shallow lateral grooves, which commence opposite the 

 posterior border of the first molar. 



The otic bullae are oval and not keeled, and have the usual bottle- 

 neck. The mastoids are convex behind, and carry above an obtuse angle 

 from the inion. 



The grinding surfaces of the molars are transverse ovals, only inter- 

 rupted by the exterior fissure. The first molar is slightly different in form, 

 being larger, and its section, when not much worn, being nearly round. Its 



