Macroclemmys temmincki, the "Alligator Turtle."–In size and general appearance much like the other Snapping Turtle, but the dorsal shields have each a strong and prominent keel, and these three series increase in size with age. The costal shields are separated from the marginals by an additional series of about four supramarginals, well shown in the illustration. The shields of the cross-shaped plastron are subject to much individual variation, small shields being frequently intercalated, or rather retained, between the usual ones, especially between the pectorals and abdominals, in the gular region, and on the narrow bridge, where the inframarginals number one to three or even more. This species inhabits, broadly speaking, the whole basin of the Mississippi and Missouri rivers.
This beast is as vicious as the other Snapping Turtle. According to Agassiz it does not withdraw its head and limbs on the approach of danger, but resorts to more active defence. It raises itself upon the legs and tail, highest behind, opens the mouth widely, and throwing out the head quickly as far as the long neck will allow, snaps the jaws forcibly upon the assailant, at the same time throwing the body forward so powerfully as often to come down to the ground when it has missed its object.
It lives mostly in the water, but makes considerable journeys overland. Both in the water and on dry land the limbs move nearly perpendicularly, and the body is raised high. On dry land a considerable part of the weight of the body is borne by the long, strong tail.
"They are as ferocious as the wildest beast of prey, but the slowness of their motions, their inability to repeat the attack immediately, their awkwardness in attempting to recover their balance when they have missed their object, their haggard look, and the hideous appearance of their gaping mouth, constitute at such times a picture as ludicrous as it is fearful and revolting. Their strength is truly wonderful. I have seen a large specimen bite off a piece of a plank more than an inch thick. They take hold of a stick with such tenacity that they may be carried for a considerable distance suspended to it free above the ground. Fishes and young ducks are their ordinary prey. They lay from twenty to forty or more round eggs only about the size of a small walnut in holes which they dig in sloping banks not far from the water" (Agassiz).
Fam. 2. Dermatemydidae.–The pectoral shields are widely separated from the marginals by inframarginals, the gular shields are very small or absent, and the tail is extremely short. Only two or three genera, with three or four species in Central America.
The plastron is composed of nine plates. In Dermatemys mawi it is large, firmly joined to the carapace, covered with eleven or more shields, and there are four inframarginals; in Staurotypus salvini of Mexico the plastron is cruciform, with the anterior lobe movable, covered with seven or more shields, according to the fusion of the anal shields and the presence or absence of the gulars; there are only two inframarginals. The pubic and ischiadic symphyses remain separate; the temporal fossa remains widely open, the postfrontals scarcely touching the parietals. There are 23 marginal shields in Staurotypus, 25 in Dermatemys, including the unpaired nuchal. The nuchal plate has a pair of rib-like processes like those of the Chelydridae, but some of the posterior costal plates, sometimes only one pair, meet in the middle line, overlying or suppressing the corresponding neural plates. The shell of these aquatic tortoises is rather flat, more or less keeled, especially in young specimens, and in the fully adult condition is about one foot in length.
Fam. 3. Cinosternidae, represented by the single genus Cinosternum, with about ten species in North and Central America, and one in Guiana. Closely allied to the two previous families, with which it agrees by the separation of the pubic and ischiadic symphyses, the presence of an ento-plastral plate, the possession of inframarginal shields (Fig. 61, 3, p. [315]), the widely open temporal fossae, and the rib-like pair of processes to the nuchal plate. It agrees with the Dermatemydidae in the interruption of the neural plates by the meeting of several pairs of the costal plates. There are 23 marginal shields; five or four shields, according to the presence or absence of the gular on the plastron, and in some species these plastral shields become, with age, more and more separated from each other by soft skin (see Fig. 75). The shape and size of the plastron differ considerably in the various species; in most of them, e.g. in C. pennsylvanicum and C. leucostomum, but not in C. odoratum, the anterior and posterior lobes are movable, with transverse soft hinges, so that the animal can completely close its shell. The skin of the legs and neck is so baggy and loose that these parts slip in, the skin rolling off, when the creature withdraws into its shell. They lay only a few–from three to five–elliptical eggs, which have a shining, glazed, and thick, but very brittle shell.
Fig. 75.–Cinosternum odoratum, young specimens. × ⅔. A and B, males; C, female.
Cinosternum odoratum, the Mud-Turtle, or Stinkpot Terrapin, so called on account of the disagreeable smell which exudes from the inguinal glands. The head is disproportionately large, with the snout rather compressed laterally, and pointed underneath, with several short barbels. The neck is long and slender. The carapace of the young is keeled, each of the neural shields being raised in the middle line; but in full-grown specimens the shell becomes quite smooth and rounded. The horny shields of the plastron are relatively largest in the young, but they soon leave ever-increasing spaces between them, which are then filled with soft skin only, which thinly covers the underlying bone. The fore- and hind-limbs, especially the latter, are extensively webbed, and are provided with five short claws. The general colour of the shell is horny brown, either uniform or with darker spots or streaks. The neck and limbs are mottled brown. The only ornamental colouring is a pair of clear yellow broad lines on each side of the head, and a similar streak on each side of the lower jaw. On the chin and upper throat are two pairs of small tentacles. The tail of the male is of about the length of the hind-limbs, while that of the female is so short that its tip scarcely reaches beyond the hinder margin of the carapace. Length of the shell of full-grown specimens between four and five inches. Very young specimens have a rather droll appearance, owing to the long and slender neck with the large head, and the humpy back.