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Family Streptomatidae

SHELL turreted or ovate; aperture angled or channeled in front; epidermis olive-hued; operculum sub-spiral; mantle not fringed. An oviparous group of five hundred species, confined to the United States except for a few West Indian species. They are found chiefly in the headwaters of streams rising in the mountains of the south central states.

Genus IO, Lea.

Shell tuberculated, with few exceptions; spire elevated; peristome flaring; canal twisted; columella smooth, concave. Few species in rivers of Tennessee and West Virginia,

The Spiny Io (I.spinosa, Lea) is the most graceful of these tuberculated fusiform shells, quite as beautiful in form as the spindle shells it resembles. Under the horny epidermis obscure purplish bands appear on an olive ground. The aperture is half as long as the shell. Length, 2 inches.

Habitat.--Virginia, Tennessee.

The prominence of the tubercles makes this species pre-eminent among American river snails, and leads to their being mistaken for marine mollusks. Specimens found in Indian graves were called "conchs" by their discovers, who argued that the tribes must have once lived near the shore. But no such shells occur in salt water.

The River Io (I. fluviatilis, Lea) is smooth or faintly knobbed, solid, greenish, stained with purple. The canal is scarcely twisted and has a rounded end. Length, 1 inch.

Habitat.--Tennessee region.

I. inermis, Anthony, is unarmed. The smooth, elegantly fusiform shell is purplish throughout. Length, 2 inches.

Habitat.--Southern States.

The Turreted Io (I. turrita, Anthony) is more elongated than I. spinosa, which it imitates in its decoration of stout spines. Two faint bands traverse each whorl. Length, 2 1/2 inches.

Habitat.--Tennessee.

I. brevis, Anthony, is stout and short, with stubby spines, five on the keel of each whorl. The canal is broad, but short. Length, 2 inches. Tennessee.

Genus ANGITREMA, Hald.

Shell conical, spiny; canal short; aperture angled; columella thickened above and below. About twelve species in Tennessee and neighbouring states.

The Knotty Angitrema (A. geniculata, Hald.) is stout, solid, almost globular, with a row of round knees on the angled shoulder of the whorls. The flaring aperture is notched at both ends. The double callus on the columella is a noticeable generic trait. Colour, yellowish olive. Length, 3/4 inch.

Habitat.--East Tennessee.

The Armed Angitrema (A. armigera, Say) is cone-shaped, with flattened and wrinkled whorls, and tubercled on a central keel. This row of knobs is buried by the revolving lip as it grows, though their presence is discoverable just below the sutures. The horny surface is often eroded at the apex; revolving red lines obscurely mark the whorls. Length, 1 inch.

Habitat.--Tennessee, Indiana, Kentucky.

Dutton's Angitrema (A. Duttoniana, Lea) is a handsome species. The elegant yellow-banded spire is conical, pointed, twice as high as wide. The long aperture is notched above and below. A row of tubercles usually follows the middle of the body whorl. Sometimes there is a keel instead. Length, 1 inch.

Habitat.--Tennessee rivers.

A. verrucosa, Raf., is ellipsoid, with blunt apex and long aperture; the lower whorl only is set with several rows of prominent warts. The outer lip is plaited within. Colour, olive brown, with white lining. Length, 2/3 inch. Ohio River.

Sub-Genus LITHASIA, Hald.

Shell small, oval or short, fusiform, smooth; columella as in Angitrema; no distinct channel. Fourteen species, inhabiting rivers of the Tennessee region.

The Dilated Lithasia (A. dilatata, Lea) spreads out its white lips below the oval, yellowish green spire, showing the brown lining. Low tubercles are sometimes seen on the shoulder. Length, 3/4 inch.

Habitat.--Tennessee.

Sub-Genus STREPHOBASIS, Lea

This small group includes a few conical shells in which the short canal is twisted under the shell.

A. curta, Hald., is greenish-brown and stoutly cylindrical, with a narrow aperture closed with a dark brown operculum. The canal is drawn under by the twisting of the columella. Length, 1 inch.

Habitat.--Tennessee.

Sub-Genus PLEUROCERA, Raf.

Shell elongated, conical, regular; aperture with short canal; columella without callus. Tryon describes eighty-two species.

The Ponderous Pleurocera (A. ponderosa, Say) is a heavy cone of flat coils, the last one keeled. The short lip canal turns to the left. The surface is olive, the lining white. Length, 2 inches.

Habitat.--Tennessee.

A. undulata, Say, has a handsome elevated spire of broad flat coils that cover a row of knobs, and so produce wavy folds below the deep suture. The white lip projects. Length, 1 1/2 inches.

Habitat.--Ohio River.

A. canaliculata, Say, is distinguished by its distinct groove on the body whorl. Variable in form and size and ranging in colour from pale green or yellow to black, yet it is distinct. It is very common at the falls. Length, 1 inch.

Habitat.--Ohio River.

Genus GONIOBASIS, Lea

Shell heavy, ovate or elongated; aperture angled in front, but without canal or notch. A large genus of one hundred and fifty species, one-half of the entire family, distributed east of the Mississippi, but well represented (as is no other genus) on the Pacific slope of the Rocky Mountains.