THE CYRENAS AND PEA SHELLS - FAMILY CYRENIDAE Shell equivalve, sub-triangular, small; hinge with two or three cardinals, and laterals; surface smooth, with periostracum; foot large, without byssus; siphons short. Sexes united in the individual; reproduction viviparous. Active bivalves, living in fresh or brackish water, believed to be derived from the marine family, Veneridx.
Genus CYRENA, Lam.
The Southern Cyrena (C. Carolinensis, Bosc.) looks like a Mactra, but has two and three divergent cardinal teeth to form the hinge, with no pit such as the surf clams have. The smooth, ventricose valves are covered with a shining, brittle epidermis, usually rubbed off at the umbones. The beaks curl over the hinge line in quite pointed hooks.
These mollusks inhabit the muddy bottoms of brackish water on sub-tropical coasts. Length, 2 or 3 inches.
Habitat.— Georgia to Texas and West Indies.
Genus SPH/ERIUM, Scop.
Shell small, oval, oblique; hinge strong; siphons separate; foot mobile, used in climbing, and as a pond snail uses its foot in floating suspended at the surface of ponds. The clam also spins a mucus thread to travel on from place to place among stems of submerged water plants. The young remain in the brood pouch until able to take care of themselves; they are then shot out through the siphon.
The Furrowed Sphere Shell (S. sulcatum, Lam.) looks like a tiny model of a Venus clam, as it is obliquely oval with beaks well forward. The mollusk is found in still water, often climbing submerged plants, and waving its two siphon tubes while the 354 The Cyrenas and Pea Shells tongue-like foot slips in and out of the broad end of the shell. Length, i inch.
Habitat.— River margins, ponds and lakes, Utah, Cal ifornia, Oregon.
Genus PISIDIUM, Pfr.
Shell somewhat pea-like in size and form, thin, smooth, with united siphons. Similar to Sphxrium, but smaller; valves not so nearly equilateral. Habits similar.
The Hidden Pea Shell (P- abditutn, Hald.) inhabits brooks and other fresh water, where it is difficult to discover, so well does its thin brown shell blend with its surroundings. Besides, it is scarcely larger than a grain of rice and may be easily mistaken for a seed of some water plant. It ranges widely, from New Mexico to the northwestern states. Modified forms have received varietal names, and many have been called new species. Several other pea shells occur in this country. There are several species in Europe. All are interesting tenants of the aquarium.
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