SACCOMYIDJ3— DIP0D0MYI1SLE— DirODOMYS. 523 



Description of the skull of Dipodomys. — As in other cases, it will be found 

 most convenient to consider the skull as a whole first, and afterward to exam- 

 ine its individual bones. An immature specimen is preferable for the latter 

 purpose, though many or most of the sutures persist to extreme old age. 



In many respects, the skull of Perognathus approaches or closely resem- 

 bles that of Dipodomys, but the family peculiarities reach their extreme 

 development only in the latter. Comparative expressions used in the following 

 paragraphs are to be considered exclusive of Perognathus, unless the contrary 

 is stated. 



The skull is much depressed ; elongated and acuminate in front ; very 

 broad behind, where the width is nearly two-thirds of the total length ; and, 

 viewed from above, presents in general a triangular shape, with the lateral 

 angles completely rounded off, and a deep emargination in the middle of the 

 base. Zygomatic arches scarcely come into this view at all ; the width of 

 the skull midway being much less than the intermastoid diameter. The out- 

 line of the zygomata is perfectly straight ; between the turgid mastoid region 

 and the expanded plate-like zygomatic process of the maxillary there stretches 

 the thread-like malar, depressed to the level of the palate. The outline of 

 the orbits is a quadrate notch between the saliences just mentioned. There 

 is no intei-orbital constriction ; were it not for the laminar zygomatic expan- 

 sion of the maxillaries and the bullous mastoids, the space between the orbits 

 would be the broadest part of the skull. The attenuate acuminate rostrum 

 springs directly opposite the broad zygomatic part of the maxillaries, and 

 extends beyond the incisors; it is at least one-third of the total length of the 

 skull. The postero-lateral aspects of the skull present enormous bulging 

 masses rounded and somewhat ovate — the extraordinarily developed mastoids. 

 The same swellings constitute also nearly all the occipital region, the median 

 line of which is a deep emargination. This character is perhaps unique ; 

 nothing like it is seen even in Perognathus ; its peculiarity is on a par with 

 the immense rostral development in a walrus for example. The resulting 

 figure, as one author has aptly remarked, bears a ludicrously close resem- 

 blance to the buttocks of the squatting human figure, the mastoids being the 

 nates, the emargination being the cleft between, and the foramen magnum 

 having an obvious suggestiveness. The whole surface of the skull is smooth, 

 and gently convex in all directions. There are no ridges or roughnesses; the 



