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Ariah/sis of Species of Ochelodon. 



A. Tail shorter than head and liody (at most ha rely as long). 



Hind ft'ct nndcr 0.70 long (usually 0.55 to 0.G5). humilis. 



B. Tail longer than head and body. 



Hind feet under 0.70 (exceptionally = 0.70). longicauda. 



Hind feet over 0.70 (rarely, if ever = 0. 70). mexicana. 



1. Ochetodon humilis, (And. and Bach.) Coues. 



Syn. Mm Jiumilix, And. and Bach., P A. N. S. P. i. 1841, 97; J. A. N. S. 



P., viii. 1843, 300 ; Q. N. A., ii. 1851, 103, pi. Ixv. (South Atlantic 



States.) 

 Hesperomys Jiumilis, Wagner, Wieg. Arch. 1843, 51. 

 Beithrodon humilis, Bd.,'M. N. A., 1857, 448. 

 Mus lecontii, And. and Bach., J. A. N. S. P. viii. 1842, 307 ; Q. N. 



A. iii. 1854, 324 (no fig.). (South Carolina.) 

 Hesperomys lecontii, Wagn., Wieg. Arch. 1843, 51. 

 Beithrodon lecontii, Le C, P. A. N. S. P. vi. 1853, 413. 



Most of which is widely different from what obtains in Ochetodon, the 

 grooving of the incisors being merely a coincidence. Nor am I satisfied 

 that the three supposed species of Beithrodon&re strictly congeneric. They 

 differ among themselves in cranial as well as external characters, to an 

 extent at least warranting subgeneric separation. In respects of palatal 

 structure and form of the anterior zygomatic root, there is a curious paral- 

 lelism with the same points in Sigmodon and Oryzomys. I will tabulate 

 some of the characters, as follows : — 



Beithrodon proper. (Tj'pe B. cuniculoides.) Anterior root of zygoma 

 deeply emarginated in front. Palate ending much behind the molar series, 

 and showing a median ridge between lateral paired excavations. Ptery- 

 goid fossfe deeply excavated, and these bones closely approximated. In- 

 cisive foramina reaching beyond the first molars. Condylar process of 

 mandible concave internally ; descending process rounded oft" inferiorly ; 

 coronoid process very oblique. Species B. cimieuloides, Walerh., P. Z. S. 

 1837, 30, and Zool. Voy. Beag. 1839, pt. ii. p. 69, pi. xxvi. (animal), pi. 

 xxxiii. figs. 2b, 2c, 2d, 2e (teeth), pi. xxxiv. figs. 2a, 2b, 2c (skull), from 

 Patagonia; and a second supposed species E. typicus, Id., P. Z. S. 1837,30; 

 Voy. Beag. 1839, pt. ii. p. 71, pi. xxxiii. fig. 4a (teeth), from La Plata. 



Suneomys, Coues, n. subg. (Tyiie Beithrodon chinchilloides.) Anterior 

 root of zygoma nearly straight in front. Palate ending nearly opposite the 

 last molars, but slightly ridged or excavated. Pterygoid fossfe shallow ; 

 these bones less approximated. Incisive foramen only reaching to first 

 molars. Condylar process of mandible flat internally ; descending process 

 angular; coronoid process nearly vertical. Species JS. (E.) chincJiilloides, 

 Waterh., Zool. Voy. Beag. 1839, pt. ii. p. 72, pi. xxvii. (animal), pi. xxxiv. 

 figs. 20a, 20b, 20c, 20d, 20e, 20f (skull and teeth), from Straits of Magellan. 

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