276 PROCEEDINGS OF THE ACADEMY OF [1895. 



The tympanic portion of the auditory bulla is more swollen and 

 of a different shape from that of any of the recent Heteromyidoe and 

 more resembles that of Dlpus. It is relatively very large and of 

 hemispherical form, with nearly equal transverse, an tero- posterior 

 and vertical diameters, and differs markedly from the elongate, 

 somewhat depressed and flask- like form characteristic of the pocket- 

 mice. The front ends of the two bulla? are quite near together, but 

 they have not the narrow prolongations, corresponding to the neck of 

 the flask, which are found not only in the pocket-mice, but also in 

 the lower Miocene genus Entoptychus, which I regard as one of the 

 forerunners of the Qeomyidce. The tympanies are in close apposi- 

 tion to the hasi-occipital and hasi-sphenoid, as in Zapus and Pero- 

 gnathus, not isolated by the fissures which occur in Dip us and Dij»>- 

 ilomi/s. The opening of the auditory n/eatus is nearly circular and 

 forms even less of a tube than in Zapus or Perognathus. 



The crowding and displacement of the cranial bones which result 

 from the great development of the mastoid bullae has already in 

 Protoptychus attained a remarkable degree, more advanced than in 

 several of the existing genera. Compared with the skull of Dipus, 

 in the fossil we find that the elements which lie in front of the mas- 

 toid are more reduced. This is due to the much greater relative size 

 of the brain in the modern genus, which has broadened the f'rontals and 

 parietals in a very striking way, and probably even the greater 

 width of the basi-occipital is due to the same factor. The Hetero- 

 myidce exhibit many differences among themselves in this respect, the 

 displacement being extreme in the Dipodomyince and very moderate 

 in the Heteromyince. Having reference to this feature only, Pro- 

 toptychus stands midway between the two sections of the family. 



The squamosal is pushed almost entirely into the orbit, but, as has 

 already been mentioned, it appears to send out a process between the 

 parietal and the mastoid, which articulates with the interparietal. The 

 sutures in this region are, however, so far obliterated as to render 

 this somewhat uncertain, but, at all events, there is at this point a 

 narrow, perfectly distinct and slightly concave temporal fossa, such 

 as exists in Heteromys and in no other member of either of the 

 modern families. In both the Dipodidce and the Heteromyidte a long 

 spur of the squamosal extends back over the auditory meatus, along 

 the line of suture between the mastoid and the tympanic. In the 

 fossil this spur is not shown, but as this region has suffered slightly 



