284 



PROCEEDINGS OF THE ACADEMY OF 



[1895. 



orbital foramen is small and perforates the maxillary far in front of 

 the zygomatic process, which is expanded and imperforate, much as 

 in the Sciuromorphs. (3) The hinder end of the incisor makes a 

 protuberance on the mandible below the condyle. (4) There are 

 large fur-lined cheek-pouches, opening externally. 



That the Geomyidce and the Heteromyidce are closely related to 

 one another is admitted by all who have studied them, almost the 

 only difference of opinion on the subject being as to whether they 

 should be grouped in one family or in two. Just wherein this 

 relationship consists is another and less simple question and it may 

 be answered in one of three ways: (1) The Geomyidce have 

 descended from the Heteromyidce; or (2) the latter have descended 

 from the former; or (3) both should be derived from a common 

 ancestor which was not sufficiently differentiated to be properly 

 referred to either family. Improbable as it may appear, the evi- 

 dence distinctly favors the first of these three alternatives. To 

 regard the manifold resembhmces between the two groups as due to 

 parallelism or convergence is a mere assumption which is without 

 any evidence to sustain it. The belief that the Geomyidce have 

 descended from the Heteromyidce involves the inference that the com- 

 paratively small size of the mastoid in the former is the result of 

 reduction from a more exaggerated development. The John Day 

 genera Entoptychtis and Pleurolicus, which differ from each other 



Fig. 5. Pleurolu us diplophysus ; cranium, 

 from above: natural size. From Cope. 



Fig. C. Pleurolicus diplophysus ; 

 occiput, from behind; natural 

 size. From Cope. 



only in the fact that the latter has rooted and the former rootless 

 molars, throw much light on this question. In skull structure they 

 arc so nearly intermediate between the two families, that the 

 reference of them to either seems somewhat arbitrary, though, on 

 the whole, they incline distinctly to the Geomyidce, as is shown in the 

 following points of resemblance: (1) The interorbital space is 

 narrower than the rostrum; (2) the palate is sloping and below the 

 level of the zygomatic arches; (3) the nasals are not produced 



