Page:The Zoologist, 3rd series, vol 2 (1878).djvu/426

402 streams that flow to the south and west, but so rare — if, indeed, present at all — as to be utterly unknown as an inhabitant of the waters flowing eastward. The cause of this is unknown, but is probably connected with the character of the water and supply of food, as the same may be said of the Brook Trout (Salmo fontinalis), which is abundant in the north-western streams of Michigan, but wanting in those whose waters mingle with Lake Huron — except those of the opposite Canadian shore.

The female Aspidonectes usually exceeds the male in point of size, attaining an extreme length of from twenty-four to twenty- eight, and, in rare instances, thirty inches, with an average length of carapace of fifteen to eighteen; while specimens of the latter are rarely found exceeding one-half the extreme size of the females, averaging usually about fourteen inches. The sternal plastron in both sexes is not ossified across its middle, and the carapace is without marginal pieces. The latter, however, is very much flattened and expanded, but unprovided with scales, covered instead with a soft derma, which has given rise to the common designation of "Soft-shelled Turtle." Nearly one-half the length of the upper shell, and more than one-fourth its breadth, is com- posed of fibrous membrane of leathery consistency, mistaken for, and erroneously termed, cartilage, by many writers. Olive-brown above, beneath it is of a dirty yellowish white, greatly mottled, streaked and dotted with black. A blunt keel along the median line slopes uniformly to the sides ; and the anterior margin is furnished with spines and tubercles, largest and most abundant in the female.

An elongated and protractile neck, and nostrils prolonged into a sort of trunk or proboscis, is likewise characteristic of the species. The jaws are trenchant, distinctly serrated on their edges, presenting the general aspects of bone, though a microscopic section reveals only the angular nucleated cells of horny tissue, while a fold of skin presents the appearance of lips. The tongue is short, triangular and soft, but not fleshy, and exhibits a peculiar surface, on which is arranged, somewhat in rows, a great number of delicate fringes resembling the filiform processes of the gills of the Menobranchus.

In common with other species of this genus, the ribs are united by, and invested with the ossification of the sub-dermal and inter- costal fibrous membrane; and hence the margins of the ribs may,