64 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



doubtful (PI. IX, Fig. 2). In the opossum the levator penis arises as a 

 direct continuation of the muscles covering the postero-ventral part of 

 the lateral bulb of the corpus cavernosum and proceeding inward turns 

 abruptly caudad and continues to the base of the glans penis. There is 

 no muscle occupying this position in Ccenolestes and none that could 

 function as a levator penis. There is another paired muscle in the 

 opossum, of which the function is not apparent, which corresponds 

 closely in position to one of the small paired muscles in Ccenolestes, the 

 most anterior and superficial one. These small muscles have been 

 designated in the figure as Nos. i, 2 and 3. No. i arises along the median 

 line on the anterior part of the bulbous urethra and continues as a 

 slender, free, and slightly thickened fascia to the anterior surface of the 

 muscle on the bulb of the corpus cavernosum. No. 2 lies partly beneath 

 No. i and overlaps No. 3, which covers the anterior side of the stalk of 

 the corpus cavernosum. There is no trace of any such muscles in 

 Macropus. From the ventral side of the corpus cavernosum near the 

 extremity of the stalk a large flattened tendon extends to an attachment 

 on the postero-internal surface of the ischium opposite the origin of the 

 gracilis muscle (PI. IX, fig. 2 , i. t.) . No such tendon is found in Didelphis, 

 but a similar attachment is normal in Macropus. A plexus of nerves, 

 blood vessels, and connective tissue emerges between the pubococcygeus 

 muscle and the ischium and reaches the corpus cavernosum and adjacent 

 glandular surfaces, assisting in attaching the very large postpubic mass 

 to the body. 



Behind the bulb of the corpus cavernosum but more dorsal in 

 position is the bulb of the corpus spongiosum, its stalk meeting the 

 bulbous urethra slightly below (ventrad) the common opening of the 

 posterior Cowper's glands. It is also enclosed in a thick layer of muscles 

 but these do not extend beyond its stalk which expands within the dorsal 

 wall of the urethra in a very thin layer. This continues to the base of the 

 glans penis where it rapidly thickens and extends to the end of the glans. 

 The corpus cavernosum continues, forming the central part of the glans, 

 practically to the pointed tip. 



The glans penis is so deeply cleft as to appear like two separate or- 

 gans. Each half is of acicular shape, narrowing at the tip to a sharp 

 point. The extreme tip is separated by a circular fold from the main 

 body of the glans as a sort of secondary glans. In front of the base of the 

 glans the urethra forks to empty into a deep groove which runs down the 

 dorsal surface of each division of the glans, terminating at the circular 

 fold' which marks off the tip of the organ. This groove is expanded in its 

 ventral part and occupies the center of the glans. In cross section in a 

 preserved specimen it appears as a transverse opening connected with 



