280 PROCEEDINGS OF THE ACADEMY OF [1895. 



is, however, only another instance of the continuity of the American 

 rodent fauna, which, scanty as it is, pursues a much more unbroken 

 course than that of Europe. Still more surprising is it to find that 

 one of the peculiarities which especially mark this family, namely, 

 the inflation of the temporal region and the concomitant reduction of 

 the adjoining cranial bones, have already attained such an advanced 

 degree. Finally, to add to the confusion, we observe that with 

 features characteristic of the Dipodidce are combined certain struc- 

 tures resembling those of the Heteromyidce, such as the dentition and 

 the shape and divisions of the mastoid bullae, while the infraorbital 

 foramen is like that of the Muridce. 



That Protoptychus is an ancestral form of the Dipodidce seems 

 abundantly clear. The long, narrow and incapacious cranium, 

 though in strong contrast with that of Dipus, is not very unlike the 

 cranium of Zapus, and these differences are just what the analogy 

 of other mammalian groups would lead us to expect. The character 

 of the infraorbital foramen offers no difficulty to this view, for its 

 gi-eat size in the modern jumping- mice cannot be a primitive feature 

 and the condition found in Protoptychus is a starting point 

 from which the modern character of the family could easily be 

 derived. The teeth are, it is true, quite different from those of any 

 of the recent genera, and yet the amount and kind of change neces- 

 sary to produce the latter are not greater than have demonstrably 

 occurred in many other families. Among the existing jumping- 

 mice there is great diversity in, the structure of the molars, which it 

 is difficult to reduce to any common plan. 



As yet no member of the family has been detected in the White 

 River formation, but in the succeeding John Day beds the line is 

 carried forward by Paciculus. This genus cannot well be directly 

 ancestral to any of the recent genera, because it has lost all the 

 premolars; but in all other respects, in which its structure is known, 

 it stands in an intermediate position between Protoptychus and 

 Dipus. The molar pattern is already very similar to that of Dipus; 

 the infraorbital foramen has become very large and its lower portion 

 Conns a distinct notch for the passage of the nerve. The interorbital 

 constriction is much deeper than in Dipus, but the cranium has 

 commenced to widen much more markedly than in Pnotoptychus. 

 The incisive foramina and posterior nares remain very much as in 



