32 AMERICAN CORBICL'LAD^. 



SPe^RIUM, ScoPOLi.' 



Pectunculus, Lister, 1685. — Musculus, Gualt. 1742. — Tellina, Linn. 

 1758. — Sphxrium, Scop. 1777. — Cardiuvi, Da Costa, 1778. — Cy- 

 clas, Brcg. 1792. — A^uz, Hdmphk. 1797. — Muscuiium, Link, 

 1807. — Cornea, Pisum, Megerle, 1811. — Corneocyclas, Fer. 

 1818. — Amesoda, Rafin. 1820. — Pisidium, Verany, 1846. — 

 Ci/cladites, Krug. 1848. 



Animal oval, lobes of the mantle simple, united posteriorly, and 

 terminating in two short syphons, joined at their base, without 

 tentacles ; mouth oval-shaped, small ; tentacles of the mouth short 

 and narrow ; gills rather broad, nearly equal, united behind the 

 foot ; foot narrow, elongated. 



Shell oval, nearly equilateral ; beaks somewhat inflated and 

 prominent ; hinge-margin narrow, with two primary teeth in each 

 valve ; lateral teeth elongated ; palleal impression simple ; liga- 

 ment external, narrow, situated on the longer portion of the shell. 



The genus Sphaerium was characterized under its present name 

 by Scopoli, in 1777 ; since that time, however, it has received 

 various denominations, and the one under which it has been most 

 generally known, that of Cyclas, was applied to it in 1792 by 

 Bruguiere. Mr. Gray revived the term of Sphaerium in 1847, 

 and his example has been followed by the conchologists of the con- 

 tinent of Europe. I was the first in this country to discard the 

 name of Cyclas for that of S2:>hseriiim. 



The species composing this genus are small bivalves inhabiting 

 rivers, lakes, streams, and still waters ; they are plentifully distri- 

 buted all over the globe, but as far as present experience goes, 

 seem to be more abundant on the northern portion of this hemi- 

 sphere than elsewhere. 



The shell is transversely oval, nearly equilateral, thin, fragile, 

 sometimes translucent, with beaks more or less raised ; its entire 

 surface is transversely striated and covered with a light epidermis 

 varying in color; the margins are rounded, obtuse or angular. 

 The interior of the valves is smooth and varies in color ; the mus- 

 cular impressions are not very distinct ; the posterior one is slightly 



' Introduct. ad. Hist. Nat. 1777, 397. 



