Neither in the Kangaroo, Phalanger, nor Da syure did the spermatozoa present a spiral head or any noticeable deviation from the characters of the spermatozoa in the smaller placental quadrupeds : those of the Dasyure have a node at the base of the head.
The corpus cavernosum penis commences by two crura(d, d, figs. 135, 136), neither of which have any i m med late attach meat to the bony pelvis. Cu vier correctly states, that in the Kangaroo the two crura of the corpus cavernosum, and the two bulbs of the corpus spongtosum, soon unite to form a single cylindrical body, having a canal which nearly follows the direction of its axis, whose parietes are equally strong and fibrous, and which contains the urethra ; so that the transverse section of the corpus cavernosum resembles a ring ; but the two lateral cavities are separated by two vertical septa which ex tend one from the central canal to the dorsum penis, the other from the central canal to the inferior wall of the penis.* In the Kangaroo and Potoroo, the erectores penis (fig. 135 d, d) arise by a thin fascia from near the lower part of the symphysis pubis, soon become fleshy, and increase in thickness as they pass outwards: each muscle then returns upon itself, at an acute bend, to grasp the erns penis, and terminates in a strong tendinous expansion at the junction of the cavernous with the bul bous structure.
The retractor penis (figs 135,136, g, g) arises in the Kangaroo from the middle of the sa crum, and divides into two muscles, behind the rectum, opposite the dilated commencement of the musculo-prostatic part of the urethra ; each division diverges to the side of the rectum, then passes to the interspace between the rec tum and roots of the penis, and along the lateral and posterior part of the penis, until it is inserted with the opposite muscle at the base of the glans.
In the Opossum and those Marsupials which, having a bifid glans, enjoy, as it were, a double coitus, there is a levator penis (f, ,f, 13G), which is not present in the Kangaroo. Each portion of this muscle takes its origin from the fascia covering the crus penis, con verges towards its fellow above the dorsum penis, diminishing as it converges, and termi nates in a common tendon inserted into the upper part of the base of the glans.
There is another powerful muscle which, though not immediately attached to the penis, must exert, in all Marsupials, so important an influence upon its erection as to merit notice here. This is the external sphincter ani, or more properly' sphincter cloaca :' it is an inch and a half in breadth in the Kangaroo and half an inch in thickness; from the back of the termination of the rectum it passes over the anal glands and sides of the base of the penis, inclosing the two bulbs with Cowper's glands and their muscles, and terminates anteriorly in a strong fascia above the dorsum penis, so as to compress against that part the venal dor sales.
This adjustment and function of the great sphincter did not escape the observation of Cowper. Speaking of the erectores penis of the Opossum, he says, " the muscles of the cavernous bodies of the penis of this creature, having no connexion with the os pubis, cannot apply the dorsum penis to the last-named bone and compress the vein of the penis, whereby to retard the refluent blood and cause an erec tion, as we have observed in other creatures ; but some large veins of the penis here take a different course, and pass through the middle parts of the bulb (crus), and are only liable to the compression made by the intumescence of the muscles (c c) that inclose them. But the chief agent in continuing the erection of the penis in this animal is the sphincter muscle of its anus, or rather cloaca; and not only the sphincter muscle of the cloaca of the male Opossum, but that of the female also, closely embraces the penis in coition, and effectually retards the refluent blood from its corpora caver nosa, by compressing the veins of the penis." The penis is bent upon itself in a sigmoid form when retracted ; with the glans concealed just within the cloacal aperture, from which it emerges, as in the Ovipara, when the penis is turgid and erect.