402 THE ZOOLOGIST. 
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, 
