NATURAL SCIENCES OF PHILADELPHIA. 307 



reduction of others, the relations with that of Geomyidse are both 

 close and clear; while Perognathus constitutes, in many respects, 

 an excellent connecting link. Numerous coincidences could be 

 pointed out, showing how the hint afforded by the presence in 

 these two families of ample external cheek pouches is borne out 

 in more essential features, notwithstanding the all but complete 

 difference in general outward appearance. 



B. Cranial Characters of Dijjodomys. 



As in other cases, it will be found most convenient to consider 

 the skull as a whole first, and afterward to examine 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 

 resembles that of Dipodomys, but the family peculiarities reach 

 their extreme development only in the latter. Comparative ex- 

 pressions 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 tri- 

 angular 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 outline of 

 the zygomata is perfectly straight ; between the turgid mastoid 

 region and the expanded plate-like zygomatic process of the max- 

 illary there stretches the needle-like molar, depressed to the level 

 of the palate. The outline of the orbits is a quadrate notch between 

 the saliences just mentioned. There is no interorbital constriction ; 

 were it not for the laminar z3 7 gomatic expansion 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 max- 

 illaries, 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 extraordinaripy developed mastoids. The same swell- 

 ings constitute also nearly all the occipital region, the median 

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



