274 PKOCEEDINGS OF THE ACADEMY OF [1895. 



appearance the occiput is quite like that of Pleurolicus diplophysus of 

 the John Day; (see Cope, Tertiary Vertehrata, PI. LXIV, 

 fig. 9 C ), but in the latter the mastoids are not so much enlarged and 

 the occiput is, consequently, wider than high. The su preoccipital 

 of Protoptych us most resembles that of the Heteromyidce ; it appears on 

 the top of the skull only as a very narrow strip and, so far as can be 

 judged from the specimen, it does not send out spurs which embrace 

 the interparietal between them, such as are found in Perognathus and 

 ( 'ricetodipus. On the plane of the occiput the su preoccipital occu- 

 pies a little more space, but even here it is very narrow from above 

 downward, nearly the whole of this surface being formed by the large 

 exoccipitals. In Dipus the spurs which enclose the interparietal are 

 very peculiarly shaped, for they are produced downward over the 

 mastoid bullae and articulate with narrow processes given off from the 

 squamosals. In Pedetes there are no such spurs from the supraocci- 

 pital and so far there is a resemblance to the fossil, but the dis- 

 tinct angle made by the occipital plane with the top of the skull 

 in the latter is heteromyine rather than dipodine in character. 

 In the Uinta genus the foramen magnum is very large in proportion 

 and of subcircular shape, while the occipital condyles are exceed- 

 ingly small, widely separated from each other and so closely 

 applied to the great bulla?, that it is difficult to see how they 

 could conveniently articulate with the cotyles of the atlas. No 

 distinct paroccipital processes are observable. Such processes 

 are distinct in Dipus and still more so in Pedetes, but in Zapus, as 

 in the Heteromyidir, they are exceedingly minute. The basioccipital 

 and basisphenoid are reduced to excessively narrow, slender rods, 

 which are more attenuated than in any of the Heteromyidce, not even 

 excepting Dipodomys. In Dipus these two bones form together a 

 wedge-shaped piece, which narrows rapidly forward; the basisphenoid 

 is thus as slender as in Protoptyehus, but the basioccipital is broader. 

 Much the same statement will apply to Perognathus. This difference 

 is due to the fact that in the fossil the largely inflated tympanic 

 bulke are of a different shape, and in particular, have a greater 

 transverse diameter proportionately than have any of the modern 

 forms mentioned. 



The mastoids are very greatly inflated and form large portions of 

 the auditory bullae, which ai - e the most conspicuous and striking 

 features of the skull. The shape of these structures has resemblance 



