695 



ON A NEW SPECIES OF REITHRODON. 



[Dec. 14. 



much longer, as sliown by the Table below, while that of Sigmodon 

 is very similar to that of 72. alstoni ; and altogether the caecum con- 

 firms the opinions expressed above as to the affinities of R. alstoni 

 with Sigmodon. On the other hand the length of the small intes- 

 tine and the proportiou that the colon and rectum bear to that 

 length, are more like those of Reithrodon than Sigmodon, 



The following Table may be useful as giving a few measurements 

 (in inches) of the lengths of the intestines in this group ; but, of 

 course, no general deductions can be drawn until a much larger 

 number of specimens has been examined. The Table has been 

 arranged in the order of the percentages tliat the colon and rectum 

 bear to the small intestine : — 



AVe thus see that R. alstoni has the short caecum of Sigmodon, 

 while it has even shorter intestines than R. chinchilloides ; so that, 

 the diameter of the canal being about the same in all, it has the 

 least extent of digestive surface of the specimens measured. The 

 Table also shows how very distinct from the other Hesperomys H. 

 {Nyctonvis) sumichrasti is, thus further confirming its separation 

 as a subgenus '. ' 



The following synopsis shows briefly the chief characters of the 

 four species of Reithrodon : — 



A. Front edge of zygoma-root concave. Fiftli digit 



short (see p. (591 ). 



a. Posterior nares contracted. Palate lengthened. 



n. Muzzle slender. Ears rather short 1. B. cuiiiculoides. 



/3. Muzzle stout. Ears longer ; tarsi shorter '1. li . fyincus. 



b. Po.sterior nares uncontracted. Palate ending oppo- 



site the last molar 3. H. ahtoiii. 



B. Front edge of zygoma-root perpendicular. Fifth 



digit long. 



c. Posterior nares and i^alate as in 2?. a/AYowi 4. E. chhic/iilloides. 



' Cf. Alston, Biol. Centr.-Am., Mamm. p. 148 (1880). 



