8 



MONOGBAPUS OF NOllTU AMERICAN RODENTIA. 



II 



large ones at baf5es of 1st, 2(1, and 5th toes, respectively; and one at con- 

 joined bases of 3(1 and 4th toes; tiiose at (he bases of all the digits, except 

 the hallux, being more or less connuent.* Pelage soft, lustrous, white 



below. 



To the foregoing characters, rather descriptive than simply definitive, 

 and indicating little else than an overgrown Hesperomys, we may add the fol- 

 lowing more diagnostic features, derived from the skull and teeth : 



Skull elongate, twice (at least) as long as wide, in spite of the divergent 

 zygomata; these do not sink to the level of the palate, and turn toward the 

 scpiamosal almost at an angle {cf. Hesperomys). Maxillar bou idary of ante- 

 orbital foramen developing no pointed process {cf. Sigmodon). Palate ending 

 as a simple emarginate or concave shelf, opposite the interspace between lost 

 and penultimate molars ; the palato-maxillary suture opposite the interspace 

 between first and middle molars {cf. any other sigmodont genus). Incisive 

 foramina very short, Foramen magnum broader than high. Auditory bullse 

 rather small; their axes very oblique to the axis of the skull. Nasal bones 

 not reaching as far back as nasal branches of intermaxillaries, which gain the 

 interorbital region. No definite bead on upper margin of orbits. Inter- 

 parietal bone subquadrate, but with a large, well-defined spur on each side. 

 Posterior aspect of skull truncate ; i. e., the occipital plane is about perpendic- 

 ular, meeting the (lattened superior surface of the skull nearly at right angles 

 (in all other genera, the coronal rounds more or less gently into the occipital 

 surface). Under jaw with long, acute, coronoid process, overtopping condyle ; 

 root of the under incisor causing a moderate protuberance on the outside 

 of the jaw, rather at the root of the condyle itself than at the notch between 

 condyle and coronoid. Teeth of the ordinary sigmodont pattern ; nearest to 

 Hesjieroimjs proper, and, as in that genus, decreasing regularly in size from first 

 to last — in tiic upper jaw at any rate ; in the lower, t he last tooth conspicuously 

 smaller than eitlicr of the other two. All the upper teeth with usually two 

 external and one internal reentrant loops of enamel (but tiie first often with 

 a supplementary internal loop, and the last often with only one external loop). 

 First and second under molars each with two internal and two external reen- 

 trant loops; last under molar with one of each. In unworn teeth, nil the 

 reentrances open, tiie saliencies correspondingly sharp and divaricating, thus 

 simulating the prismatic structure of ArvicoUna ; in old teetii, however, these 



•The tubercles at bases of all the exterior lingers and toea show a tendency to develop little 

 accessory tubercles upon tlioir outer faces. 



