CHARACTERS OF SCARTURUS AND OTHER JERBOAS. 675 



for instance, the hairs of the tuft are long and show a marked 

 tendency to a bilateral, distichous, or feather-like arrangement, 

 apparently to give support and prevent the end sinking into soft 

 sand when used as a prop ; a.nd probably for use as a guide-mark 

 in the dark, the tuft is usually conspicuously black or brown and 

 white, the distal half being white and the proximal black or deep 

 brown, and frequently additional white hairs precede the dark- 

 coloured portion. The size of the tuft, however, varies even 

 within specific limits. It is, for example, very large in Allactaga 

 saliens Gm. (= jaculus Pall.) and quite small in Alactagulus 

 acontion Pall, and Allactaga elater Pall., judging from Lichten- 

 .stein's figures. In the two first-mentioned, moreover, it is 

 conspicuously coloured, whereas in the last it appears to be uni- 

 formly or nearly uniformly brown. In Euohoreutes, according to 

 Sclater, it is moderately developed and bicoloured, but " the 

 hairs do not seem to be arranged in so distichous a manner as in 

 Alactaga." 



Two genera, however, of this family — namely, Pygeretm%is and 

 Cardiocranms—have the tail of a very difiierent shape. It is 

 narrow and cylindrical at the base, then broadened, depressed, 

 and lanceolate, with the apex attenuate, but not tufted at the 

 end as in typical Jerboas. Thus two distinct types of tail are 

 found in the Jaculidse. 



In Pedetes the tail is long and bushy throughout, being 

 uniformly and thickly covered with long hairs from base to 

 apex. 



Summary of the Characters of the Tails. 



a. Tail subcylindrical or subquadrate. 



h. Tail uniformly and thickly covered throughout with long hairs... Pedetes. 

 V . Tail covered for the most part with short hairs, often tufted at 

 the end. 

 c. Tail Covered with short hairs and rows of scales, at most 



pencilled at the end Sicista, Zapus. 



c' . Tail covered with hairs, but not visibly scaly, with larger or 



smaller terminal tuft Jaculus, Scirtopoda, 



Scarturus , Allactaga, JSucJioreutes. 

 a. Tail broad, flattened, and lanceolate, covered with short hairs, 



apicallj' pencilled, not visibly scaly Fygeretmus, Cardiocranius. 



Anus and External Genitalia. 



In Zapus the anus, as in most Rodents, opens in the centre of 

 an area of naked skin above the genital orifice, the two orifices 

 being separately visible on superficial inspection. In the female 

 the vulva is, however, just below the anus and the naked skin 

 surrounding it is continuous above with that surrounding the 

 anus. There is no elongated, conical, preputial excrescence and 

 no visible external clitoris. In the male the penis, normally 

 retrospective, is separated by a fi'inge of hair from the anus 



Proc. Zool. Soc— 1922, No. XLYI. 46 



