1895.] NATURAL SCIENCES OF PHILADELPHIA. 275 



both to the Heteromyidas and the Dipodidce. The relative size of 

 the mastoids is about the same as in Dipus and Pedetes, but they have 

 a less extension vertically and a greater one antero-posteriorly than 

 in those genera. In Zapus the mastoids are but moderately inflated, 

 and in Heteromys they are in much the same condition, so as not to 

 form conspicuous parts of the cranial walls. In Perognathus these 

 bones are much more swollen and conspicuous, but they form com- 

 paratively little of the top of the cranium, do not bulge out strongly 

 from the sides of the skull and project but slightly behind the plane of 

 the occiput. In this genus the mastoid articulates extensively with 

 the parietal and helps to give it its' characteristic pentagonal shape. 

 In Oricetodipus the mastoids are still more swollen and project so 

 decidedly backward that the proper occipital surface appears like an 

 emargination. In the Difiodomyince the mastoids are inflated to a 

 most extraordinary degree, reducing all the other cranial bones of 

 that region to mere strips. In Protoptych us these bones are dilated 

 much more than in either Perognathus or Oricetodipus, though not 

 approximating the enormous size which they attain in Dipodomi/s, 

 and form very conspicuous prominences, which project both 

 laterally and posteriorly. They do not, however, articulate with the 

 parietals, from which they appear to be separated by the narrow 

 prolongations of the squamosals which form the temporal foss:e. The 

 parieto- mastoid articulation occurs in the modern forms of both the 

 Dipodidce and the Heteromyidce, even in those genera in which the 

 mastoid is only moderately inflated. The construction of the mas- 

 toid bulhe in the fossil has most resemblance to that of the lletero- 

 myidce. In Dipus and Pedetes this structure, is high vertically, not 

 much elongated antero-posteriorly and not so divided into chambers 

 that the division is clearly visible from the outer side. In the fossil, 

 as in Oricetodipus, the mastoid bulla is laterally compressed and 

 more extended antero-posteriorly than laterally; it is divided by 

 partial septa into chambers, two of which are plainly shown, even 

 externally, being bounded by deep grooves. Of these two chambers, 

 the hinder one is short, high and narrow and lies behind the audi- 

 tory meatus, while the anterior one is longer, lower and broader and 

 lies above the auditory meatus, the two divisions meeting at nearly a 

 right angle. In Dipodomys the arrangement is similiar, but the 

 anterior chamber is much more strongly inflated and, in particular, 

 is very much wider. 

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