averages from three to four inches in length in a relaxed condition, and nearly half an inch in diameter. Its posterior extremity is provided with two large, soft, diverging and erectile bulbs, the erectile tissue of which extends through the whole length of the corpora cavernosa.
The peritoneal canals, which in other species penetrate nearly to the male organ, in the Aspidonectes do not reach one-third the length of it. The testes seem disproportionately large in smaller individuals, those of a reptile scarcely nine inches in length equalling those of a specimen fully fourteen inches over all. They measured one and one half inches in length, by from one-fourth to to one-third of an inch in thickness, having the elongated form of the organs of pennibranchiate reptiles rather than the round one common to the Testudinatæ. Both the epididymis and the vas-deferens are very long and convoluted.
Of the peculiarities of the muscular system, those of the retractors of the neck present the most interesting differences. Cuvier describes the retrahens capitis et colli as arising by its longer part from the ribs of the fifth and sixth dorsal vertebrae, within the carapace, and, passing in the interval between the lungs on the sides of the anterior part of the neck, is inserted by means of fasciculi to the transverse apophyses of the third, fourth, and fifth cervical vertebrae, the long fasciculus passing to the head to be inserted on the basilar process of the occipital bone ; the shorter portion, arising from the fourth and fifth dorsal vertebrae beneath the articulation passes for insertion into the side of the sixth cervical. But in the Aspidonectes the retrahens cervicis is a single muscle on the side of the neck, and at the anterior attach- ment, but in its posterior insertion becomes a triceps. Attached to the side of the first cervical vertebrae it follows by its side to the thorax, where it divides into three separate portions, which, how- ever, remain side by side in the anterior part of the cavity and between the lungs, there dividing, one part passing back alongside the vertebral column, through the pelvis, to be inserted into the posterior portion of the fourth caudal vertebrae ; the other two portions diverge laterally, one being inserted into the intercostal space of the sixth and seventh ribs, and the other into the outer part of the like space of the seventh and eighth. This muscle replaces the retrahens capitis collique described by Ludwig Heinrich Bojanus.
