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Show 696 ON A NEW SPECIES OF REITHRODON. [D much longer, as shown by the Table below, while that of Sigmodon is very similar to that of R. alstoni; and altogether the caecum confirms the opinions expressed above as to the affinities of R. alstoni with Sigmodon. On the other hand the length of the small intestine and the proportion 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 that the colon and rectum bear to the small intestine :- Name. Reithrodon chinchilloides, Waterh. Hesperomys sumichrasti, De Sauss. Hesperomys angouya, Desm Length of head and body. (a) 51 5*3 4*7 4-7 5-1 5-5 4-7 51 4-7 5-3 Whole intestine. (b) 24*3 21*3 27*7 328 33*5 42 5 31*9 35*9 27*9 36*4 Small intestine from pylorus to caecum. t<0 14*5 13-4 17*4 20*5 22*9 29-5 22*4 25-6 22-0 30-0 Colon and rectum. (d) 9*8 7-9 10*3 12*3 10*6 13*0 9-5 103 5-9 64 Percentage of d to c. (e) 67 59 59 59 46 44 42 40 26 21 Caecum. (/) 3-55 1*85 355 2*8 215 2-35 1-9 1-95 107 1*05 W e 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. (Nyctomys) sumichrasti is, thus further confirming its separation as a subgenus 1. The following synopsis shows briefly the chief characters of the four species of Reithrodon :- A. Front edge of zygoma-root concave. Fifth digit short (see p. 691). a. Posterior nares contracted. Palate lengthened. a. Muzzle slender. Ears rather short (3. Muzzle stout. Ears longer; tarsi shorter b. Posterior nares uncontracted. Palate ending opposite the last molar B. Front edge of zygoma-root perpendicular. Fifth digit long. c. Posterior nares and palate as in B. alstoni 1. B. cuniculoides. 2. B. typicus. 3. B. alstoni. 4. B. chinchilloides. 1 Cf. Alston, Biol. Centr.-Am., M a m m . p. 148 (1880). |