MURIDiE— SIGMODONTBS— vSIGMODON. 31 



or alluded to; these characters appearing the more strongly, because, in the 

 course of time, the intermediate connecting stages of change have become 

 effaced. On the other hand, we discard the names occidentalism mexicana, and 

 micropus, because these terms express indefinite, more or less intangible, and 

 fluctuating phases of development, which are of less importance, since 

 the links that bind the forms to their respective branches of Neotoma are 

 perfectly plain. Moreover, the first-named species all rest upon very broad — 

 primary, so to speak — geographical, and therefore climatic, conditions; while 

 the last-named indicate only the operation of restricted local influences. 



Genus SIGMODON, Say & Orel. 



Sigmodon, Say & Ord, Joiirn. Acad. Nat. Sci. Phila. iv, 1825, 352— Aud. & Bacii., Q. N. A. i, 1849, 227.— 

 Baird, M. N. A. 1857, 501.— Couks, Proc. Acad. Nat. Sci. Phila. 1874, 175; and of authors 

 generally. 



Arvicola, sp., Aud. & Bach. ; Harlan ; Godman. 



Besperomys (Deilemys), sp., De Saussure, Rev. & Mag. Zool. 1800. (" S. toltecus".) 



Chars. — Skull and dentition much as in the sigmodont Mures at large. 

 Length of skull less than twice the width of the widely divergent zygomata; 

 these arches set very obliquely to the axis of the skull, and not dipping down 

 to the level of the palate. Plate of maxillary forming the outer wall of the 

 anteorbital foramen with a concave front border, because of a prominent pro- 

 cess that nearly divides the lower slit-like aperture of the foramen from the 

 upper oval part. A prominent raised bead at the supraorbital border, running 

 thence backward obliquely outward on the temples to the occiput. Palate 

 ending behind, opposite the posterior border of the last molars, with a slight 

 emargination divided in two by an azygos median process of bone, and exca- 

 vated on each side of this, with a minute foramen at the bottom of the exca- 

 vation behind. Incisive foramina long. Tympanic bullae comparatively small. 

 Jaw with the coronoid process overtopping the condyle ; the root of the under 

 incisor forming a protuberance on its outside near the notch between condyle 

 and coronoid. Descending process rather large and flat, with incurved under 

 border. Pelage hispid, from abundance of large bristly hairs. General form 

 stout, somewhat arvicoline. Profile of head strongly convex ; muzzle short, 

 obtuse, hairy, except the nasal pad. Tail from two-thirds to three-fourths as 

 long as the head and body ; generally less than the body alone. Ears large, orbic- 

 ular, sub-naked outside, hirsute inside. Antitragus well developed, valvular. 

 Fore feet small, not half as long as the hinder; palms naked, 5-tuberculate (two 

 large tubercles abreast, posterior, one at base of 2d and 5th fingers respectively, 

 one at base of 3d and 4th together) ; thumb rudimentary, with an obtuse, Hat- 



