408 Mil. 11. I. roGOCK ON Tllli EXTERNAL 



walls nntl is armed at the l)ottoui with two lon_e^ spikes, but each 

 of these instead of being simple is divided into a paii- of snbequal 

 branches risinji,- from a common base. There are thus four spikes, 

 a. peculiarity not recorded elsewhere in the Hystricomorpha. 

 (Text-fig. 22, A, B.) 



The prepuce of. Ctenomys is closer to the iinus than in Octodon 

 and the penis itself is thinner, and when viewed from behind or 

 below is seen to be tolerably evenly attenuated to the nan-owed 

 apex, but from the side its distal end is a little expnnded behind 

 the pointed tip. It is beset with spicules, and the pouch is 

 corrugated and armed with a single pair of long spikes, as in 

 Dasyprocta. (Text-fig. 22, D-F.) 



In Dasj/jyrocta the glans of the penis, figured by Tullberg, is 

 subcylindrical, with a bluntly rounded apex, and is beset, espe- 

 cially beneath, with minute recurved sharp papillae ; but on each 

 side of it there is a long lustrous horny blade-like lamina, attached 

 by one edge to the epithelium of the glans, the other edge being- 

 free and finely serrate. This plate is capable of erection, and in 

 this state the free edge stands away from the glans like a ridge. 

 Presumably its function is to fix temporarily the penis in the 

 vagina during copulation. (Text-fig. 23, L.) 



The penis of Coelogenys, described by Owen and others, 

 tolerably closely resembles that of Dasyprocta, except that the 

 horny plate is represented by a short, hard, erectile plate, the free 

 edge of which is armed with about five comparatively large, 

 widely-spaced, thorn-like teeth. In both genera there is a pair 

 of long spikes in the glandular pouch. (Text-fig. 23, I, K.) 



In Cavia porcellus and Galea liUoralis the prepuce is at the 

 lower extremity of the naked subcaudal tract. The glans penis 

 is subcylindrical and apically rounded. On each side of it 

 towards the apex there is a long narrow flap of epithelium, the 

 edge of which is serrulate with soft papillfe, and above this there 

 are a few shorter smaller flaps. The longer larger flap, no doubt, 

 represents the horny lustrous plate on the penis of Dasyprocta, 

 but it is quite soft and pliable, not rigid and horny as in that 

 ;genus. The glandular pouch is deep and provided with a pair 

 of long spikes lying between two strongly developed i-idges on 

 the upper or anterior wall of the pouch. (Text-fig. 23, F-H, M.) 



The penis of Dolichotis projects backwards from the low^er 

 portion of the naked area of skin extending up to the root of the 

 tail. The prepuce is moderately long, but naked. The glans 

 penis is slightly nan-owed apically, its surface is reticulated and 

 pitted, and the genito- urinary orifice, just beneath the tip of the 

 baculum, is lai-ge. The deep, proximally narrowed, glandular 

 pouch is lined with rows of elongated papillae representing the 

 laminate ridges of other genera, but there is a pair of low 

 thickened ridges, corresponding to the large iddges of Cavia and 

 Galea^ passing down the anterior or upper wall. No spikes are 

 present and no trace of the erectile papillate lateral ridge seen in 

 Cavia and Dasyprocta was detected. (Text-fig. 23, A-D.) 



