MYOMOEPHA. 869 



Measurements. 



M. 



Length of skull, including mastoid bulla 0340 



Length from front of premaxillary to Pm. iv 0145 



Length of otic and mastoid bullae 0100 



Interorbital width... 0050 



Width of muzzle at middle 0080 



Width between the Pm. iv s 0028 



The exposure of the supraoccipital is wide and subquadrate, resembling 

 only that of the Pleurolicus diplophysus among the rodents of this group. 

 The temporal ridges converge gradually in a straight line posteriorly to the 

 supi-aoccipital bone and then diverge without coming in contact. 



The John Day River of Oregon. 



Pleurolicus diplophvsus Cope. 



Bulletin U. S. Geol. Survey Terrs., IV, p. 381. American Naturalist, 1883, p. 167, fig. 16 c, d. 



Plate LXIV; fig. 9. 



A fragment of a skull which includes all posterior to the interorbital 

 region is all that represents this species. The maxillary bones and teeth 

 are lost. The interorbital region is concave, there being on each side a low 

 angular ridge. These ridges continue into the temporal ridges, which have 

 the same character as in the P. leptophnjs. They are straight and converge 

 to the anterior part of the supraoccipital bone, where they do not come in 

 contact, but diverge to end at the inion. The supraoccipital a triangle 

 with obtuse apex and as wide as long. The tympanic tube is quite short, 

 the shortest found in this group. The mastoid bulla is large, a little exceed- 

 ing the otic, and has a general convex external face, which is not divided 

 into two planes, an external and a posterior, by a sti'aight angle, as in most 

 -of the allied species. There is a small portion homologous with the external 

 face which turns inwards and passes under the squamosal, leaving a consider- 

 able fissure-like foramen, which is wanting in most of the other species. The 

 postsquamosal foramen is large, and the hammer-shaped bone very distinct, 

 sending its posttympanic process to the meatus. The otic bulla is cut off from 

 the mastoid by a deep oblique fissure. One end of the fissure is marked by 

 the posttympanic process, and the other by the paroceipital. The bulla is 

 -compressed and flattened on both inner and external sides. A strong ril » oon- 



