412 MR. R. I. POCOCK OX TUE EXTERNAL 



In Chinchilla the penis lies some distance in front of and 

 below the anus on the pubic area between the hind legs, and is 

 represented externally by a long hairy prepuce projecting down- 

 wtirds and slightly backwards. When extended from the prepuce 

 it is seen to be subcylindrical, with a slightly expanded, truncated 

 extremity and with the epithelium beset with minute spicules. 

 The genito-urinary orifice is terminal, and immediately beneath 

 it is the larger orifice of the glandular pouch, which has longi- 

 tudinally laminate walls but no spikes at the bottom. Thus, 

 apart from its elongated prepuce, projecting freely from the 

 pubic integument, the penis of Chinchilla structurally resembles 

 that of typical Hystricomorph Rodents. (Text-fig. 25, A-D.) 



The penis of Lagustomus, however, difl['ers in one very important 

 point, and is unique so far as my observations go. As in 

 Chi7ichiUa, it in represented externally by a long, pendulous, hairy 

 prepuce, which, however, is abdominal in position, being set 

 much further forwards thaia in Chinchilla. But the penis itself, 

 sheathed in the prepuce, is an exceedingly long and slender rod, 

 apically attenuated so as to be almost filiform at the tip and 

 much thinner than the lumen, of the preputial sheath. The 

 pouch at the tip of the penis appears to have aborted. (Text- 

 tig. 25, E-H.) 



Tullberg described the glans of the penis in a young example 

 as '"diinnund stark zugespitz, fast lanzettenformig." 



The following tabulation, setting foi'tli the principal variations 

 in the structure of the penis, is based entirely upon my own 

 observations. lix many respects it would differ materially from 

 a table based upon the recorded observations of Tullberg : — 



a. Glans of penis exceedinglj- long and slender, without glandular 

 poucli : elongated prepuce situated far forwards on tlie 



abdomen Lagostomvs. 



a'. Glans of penis comparatively short and stout, with glandular 

 pouch ; prepuce long or short, but pelvic in position and 

 comparatively near the anus. 

 b. Glandular pouch with a pair of spikes at the bottom. 



c. Spikes very short Acanthion, Erethizon. 



c'. Spikes very long. 



d. Spikes biramous Octodon. 



d'. Spikes simple, undivided. 



e. Spiniform papilla? on penis not an-anged in deiinite 

 rows ; a soft pointed lappet below urino-genital 



orifice, as in Octodon .. Cfenomys. 



e'. At least one long row of spiniform papilla on an 

 erectile ridge on side of penis ; no lappet below 

 orifice of penis. 

 f. The erectile ridge with its papillae quite soft and 



uncornified Galea, Cavia. 



f. The erectile ridge and its papillae forming a hard 

 horny plate. 



g. Horny plate long with serrulate free edge Dasyprocta. 



g' . Horny plate short, with about five lai-ger spines 



on its free edge Coelogenys. 



h' . No spikes at bottom of pouch. 

 /(. Prepuce, when penis is contracted, withdrawn into sac, 



involving the anus Hydrochoerus. 



