CHARACTEKS OF SOME HVSTKICOMORPII RODE>"TS. 405 



outside the limits of tlie Dasyproctidte and Caviidje, namely, in 

 Ociodon and Ctenomys, where they ai-e of large size, and in 

 Acanthion and Erethizon, where they are short. I do not know 

 what the explanation of these differences may be. Probably, 

 however, these spikes are only fvdly developed in mature males ; 

 but, since my examples of Dolichotis and Hydrochoerus were fully 

 mature, it also occurs to me as a possibility that they may be 

 broken off during copulation and reproduced subsequently. Also 

 I do not know what their function may be, unless it is to 

 penetrate the orifices of the Fallopian tubes, either to make a 

 passage for the semen or, if shed, to block the oiifices after the 

 introduction of the semen. However that may be, one thing is 

 clear, namely, that the presence of these spikes in such widely 

 separated genera as Acanthion, Erethizon, and Octodon does away 

 with their importance as evidence of kinsliip between such genera 

 as Dasyprocta, Coelogenys, and Cavia. 



One other point to note is that the testes never pass into a 

 scrotum in the Hystiicomorpha. 



The penis of Hystrix and Acanthion is retrospective, when at 

 rest, being bent backwards on itself so that the prepuce forms a 

 swelling a little beneath the anus. When erected the penis pro- 

 trudes a long way from the prepuce and projects forwar ds. It is 

 furnished with an apical baculum. The orifice, when dilated, 

 reveals two apertures, an upper and smaller one immediately 

 beneath the tip of the baculum and a lower one forming a trans- 

 verse slit which leads into a tolerably deep glandular pouch, with 

 puckered walls and a pair of small spikes at the bottom. This 

 pouch can be evaginated, and in this condition it projects well 

 beyond the normal tip of the penis. It can be withdrawn by the 

 action of a couple of tendons running backwards along the lower 

 half of the penis beneath the urethral canal. The epithelium of 

 this glandular pouch and of the penis itself is beset with minute 

 spicules, but the epithelium at the bottom of the pouch round 

 the tooth-like spikes is smooth. (Text-fig. 20, A-F.) 



According to Parsons (P. Z. S. 1894, pp. 251-296) the male 

 genitalia oi Atherura resemble those of Hystrix. 



In the male of Coendu novoi-hispanicv the penis opens at the 

 lower extremity of the naked area of skin common to it and the 

 anus, as described lielow (p. 417). When at rest the penis is 

 retrospective and completely withdrawn within a short naked 

 prepuce. The structure of the penis is almost exactly the same as 

 in Acanthion longicauda. The orifices of the urethra and of the 

 glandular sac are normally concealed by the epithelium at the 

 tip of the glans penis, but when this is spread the orifice of the 

 ui-ethra is exposed just beneath the tip of the baculum, with 

 that of the glandular pouch behind it. This pouch has longi- 

 tudinally corrugated walls and a pair of small spikes at the 

 bottom. (Text-fig. 20, G-K.) 



In Myocastor the penis opens retrospectively a little distance 

 below and in front of the anus and is normally entirely 



