380 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol. YI. 



Superciliary borders sliarp, vertical, continued into two straight temporal 

 angles, which form a V. 



Premolars narrow ; size of E. planifrons E. lambdoideus. 



AA. Thickened ridge on the superior side of each supercilium ; front narrower. 



Superciliary ridges soon discontinued ; size of E. planifrons E. cavifrons. 



AAA. Superciliary ridges much thickened, soon uniting, and closing the frontal 

 groove behind. Front narrowest. 

 Premolar widened at the base ; size of E. planifrons E. crassiramis. 



Some differences in the form of the mastoid bone may be observed in 

 species of this genus. Thus it is flat beliind, and bears a well-marked 

 "lateral occipital" angle in E. planifrons and E. lambdoideus, while in 

 the remaining species it is convex, and with the angle little apparent. 

 In some specimens the loss of the hammer-shaped squamosal dismem- 

 berment, which I will call the posttympanic bone, gives a deceptive ex- 

 tension forwards to the mastoid. 



Parts of more than a hundred individuals of Entoptychus are in my 

 collection. 



PLEUEOLICUS Cope. 



Paleoutological Bulletin, No. 30, p. 3, December, 1878; Proceedings Amer. Philosoph. 



Society, 1878 (1879), p. 66. 



Family Saccomyidce. Superior molars rooted and short-crowned. The 

 crowns with a lateral fissure bordered with an inflection of the enamel 

 sheath, extending to their bases. In the superior molars this inflection 

 is on the external side, and does not divide the crown. Superior inci- 

 sors not grooved. 



This genus is curiously near to the existing Reteromys and Perogna- 

 tlms, the two genera of Saccomyidce with rooted molars. The former 

 difi'ers in having the molars divided into two columns, each of which is 

 sheathed in enamel, while Perognathus only diflers, so far as I am aware, 

 in having the superior incisors grooved. It is also very nearly related 

 to Entoptfyclms, and two of the species correspond in various respects 

 with two of those of that genus. In view of the fact that most of the 

 specimens of the P. sulcifrons are old individuals with well worn molars, 

 the idea occurred to me that the rooted character of the molars might 

 be common to the species of Entoptyclms, but that it might not appear 

 until long use had worn away most of the crown, and the protrusion 

 had ceased. Examination of the bases of the long molars of E. plani- 

 frons did not reveal any roots. It is also opposed to this view that the 

 maxillary bone in the Pleurolici has little depth below the orbital fossa, 

 appropriately to the short-rooted molars, while the depth is considerable 

 in the typical EntoptycM, though there is a complete gradation in this 

 respect. But I have demonstrated satisfactorily that PleuroUcus is a 

 distinct genus by observations on the P. leptophrys. Some of my indi- 

 viduals of this species are young, with the crowns of the molars little 

 worn, yet the roots diverge immediately on entering the alveolus, on 

 all the molars. In the species of PleuroUcus the lateral fissure of the 



