THE CRANIAL AND DENTAL CHARACTERS OF GEOMYID^. 



By Dr. Elliott Coues, U. S. A. 



I 1=1. O ^. P ^. M^- _ ^ _ 1^ _ 20 

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The skull and teeth of the two genera, Geomys and Thomomys, which 

 compose this family, are so similar that they may be described for the 

 most part in identical terms. The generic characters will be pointed 

 out in the sequel; and some of the remarkable peculiarities of the cra- 

 nium of the most closely allied family, Saccomyidw, will be noted. 



In its massiveness aud angularity the skull of the Geo my idee differs 

 altogether from that of the 8accomyid(e, in which the crauium is singu- 

 larly papery and bullous, with few augles, and quite closely resembles 

 an arvicoline type. The jaws are remarkably strong; the incisors im- 

 mense ; the zygomata flaring ; the occipital region is extensive ; the 

 palate proper is contracted and at the same time prolonged downward; 

 there is a long arched interval between molars and incisors. On a plane 

 surface the skull without the lower jaw rests level upon the molars and 

 incisors, no other points touching the support. The molars are all 

 rootless and perennial. The inferior incisors traverse the w^hole jaw. 

 The superior incisors are semicircular. Xo anteorbital foramen occupies 

 a usual site. The complex temporal bone is inordinately enlarged in all 

 its elements, but especially the squamosal, which represents most of the 

 cerebral rooting at expense of tiie reduced parietals. The malar is 

 merely a short spHnt ; there is an osseous tubular meatus auditorius. 

 There are no orbital processes ; the interorbital constriction is narrower 

 than the rostrum ; the latter is more than a third of the length of the 

 whole skull. Such are some of the general features, from which we may 

 proceed to details — first of configuration of the whole, afterward of char- 

 acters of individual bones. 



Viewed from above, rather less than the posterior two-thirds of the 

 skull ]3resents a subquadrilateral figure, from which the rostrum pro- 

 trudes in front. The greatest width is opposite the fore part of the 

 zygomata, in most cases, though specimens differ in this respect, owing 

 to a variable curve of these parts. In adult Geomys, the case is as stated, 

 the zygomata converging a little backward in a nearly straight line, so 

 that posteriorly their width apart is little if any greater than the iuter- 

 mastoid diameter of the skull. In Thomomys there is a more decided 

 outward convexity of these arches, and their greatest width apart is 

 nearly at their middle — if anything posterior to this, and at any rate the 

 width here decidedly surpasses the iutermastoid diameter. In front, the 

 zygomatic plates of the maxillaries start out at nearly a right angle 

 with the long axis of the skull ; behind, the zygomata curve rather ab- 

 ruptly in to the squamosal. There is a deep abrupt emargination be- 

 hind the posterior root of the zygomata, between this and the postero- 

 lateral corner of the skull ; in the recess, the tubular bony meatus audi- 

 torius appears protruding in this view. The lambdoidal crest, forming 

 the posterior boundary of the skull, is a slight curve, more or less irreg- 

 ular; most of it is squamosal, for the occipital bone rises to this crest 

 for only a short distance. The narrowest part of the skull is between 

 the orbits, where the width is less than the diameter of the rostrum. 

 The irregularly pyriform figures, circumscribed by the zygomata and 



