810 TORTOISE testudinata except the marine species or turtles that is, the amydcB of Oppel as distinguished from his chelonii. Their general characters have been given under TESTUDINATA. The suborder amydce, according to Prof. Agassiz, comprises the following seven families, not equally related to each other : trionychidw or soft tortoises, chelyoidw (the matamata), Tiy- draspididcB, chelydroidce or snappers, cinoster- noidce or mud tortoises, emydoidce or terrapins, and testudinina or land tortoises. Of the very numerous species here included, space will permit the mention of only a few of the typi- cal forms in the above order of families. Du- me"ril and Bibron divide the amydce, according to habitat, into chersites or land tortoises, cor- responding to testudinina, and elodites or marsh tortoises, including all the other families except the triony chides, which form their pota- mites or river tortoises. Though some pass nearly all their life in the water, none are en- tirely aquatic, and none can swim unsupported for great distances; when in the water, they usually remain at the bottom, and seldom swim freely except when alarmed or seeking to leave it. Their locomotion is a kind of walking, the weight being about equally dis- tributed on the front and hind limbs, which have nearly the same development, the motions of each pair alternating with each other. The shield or carapace is more symmetrical than in the turtles ; the feet are always distinct from the legs, and movable upon them ; the toes are either separate and short, or united by a web capable of expansion and contraction; the limbs can generally be withdrawn under the carapace, and the head wholly or partially. The tortoises rank higher in the order than the turtles. In the trionychidce the carapace is flat, thin, and oval, and very incomplete, the ribs united only on the median line, and ex- tending thence to the margin like spokes of a wheel ; it is covered with a tough skin, flexible on the margins ; neck long and flexible ; head pointed, and terminating in a long leathery snout ; jaws covered with a horny sheath, and the lips fleshy ; feet short, broad, and strong, five-toed and fully webbed, three of the toes with claws ; limbs only partially retractile and moving horizontally ; skin loose and free about the neck and limbs. The oldest geological deposit in which any of this family has been discovered is the greensand of New Jersey. They are active species, preferring the muddy bottom of shallow water, sometimes lying con- cealed in the mud with only a part of the head exposed, taking breath from time to time by stretching up their long neck and raising the tip of the snout above the surface ; they can remain under water more than half an hour at a time, rarely going on land, where their movements are awkward ; in the water they move rapidly, striking suddenly at objects by means of their long neck ; they prey principal- ly on fish, seizing also frogs, small birds, and young alligators and lizards ; the species found in the Nile is very destructive to young croco- diles ; they have been known to attack persons bathing. They are very wary, but are fre- quently caught on hooks baited with a live fish; their flesh is highly esteemed. Their eggs are numerous, spherical, and very brittle ; they are deposited on sandy shores near the water in April and May, and the young appear in July. The genus trionyx (Wagler), equiva- lent to cryptopus (Dum. and Bibr.), is peculiar to Asia and Africa, the species of this country formerly referred to it belonging to the genera aspidonectes (Wagler), platypeltis (Fitz.), and amyda (Ag.). The common soft-shelled tor- toise of the northern states (A. spinifer, Ag.) attains a length of 14 in. ; it is yellowish brown, beneath white, 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 ; it is found from Lake Champlain to Pennsylvania and west to the Missouri and Mississippi rivers; its flesh is very delicate. Other species are found in the southwestern states. Very large species of this family were brought here from west- ern equatorial Africa by Mr. Du Chaillu. The chelyoidce somewhat resemble the preceding family, but the head and neck are more re- tractile and furnished with numerous mem- branous fringes and lobes of singular form. There is only the single genus cTielys (Dum.), and a single species, the matamata ( C. matama- ta, Dum.), attaining a length of 2 or 3 ft. ; it inhabits the stagnant waters of tropical South America, feeding on fish; it is captured for its excellent flesh. The hydraspidida, contain- ing the genera platemys, podocnemys, &c., were united to the chelyoids by J. E. Gray, the two forming the elodites pleuroderes of Dume'ril and Bibron. The neck is long, the head re- tractile or bent laterally under the shield ; in some the skull presents the union of the tem- poral and parietal bones to form a broad roof over the temporal region, as in marine turtles, combining thus the family characters of the two suborders. Prof. Agassiz thinks that po- docnemys will be found to agree more closely with the earlier geological types than with any other, and that the group of pleuroderes bears the same relation to other testudinates that the marsupials do to ordinary mammals. The sexual differences are so great that they have been mistaken for specific; the tail of the male is much the longer, and in this sex there are sharp asperities between the joints of the hind legs ; the colors are also different. This group is foreign to the United States, and mostly South American, a few being found in Africa and Madagascar. The chelydroida, described in the article SNAPPING TURTLE, are thoroughly aquatic, and the lowest of the amydcR except the preceding families; they are characterized by their keeled back, ser- rated margin, broad, flat, and imperfectly re- tractile head, narrow and cross-like sternum, and large tail. The cinosternoidce have a long
