Sphcerium. \ PELECYPODA. 933 



Fam. SPHJERIIDJE, Dall. 



Cyclndidce, Clark. 



Animal having the mantle with 1 siphon or with 2 free siphons, 

 which have simple orifices. It is hermaphrodite ; the embryos are 

 incubated in the external gill-plate. 



Shell small, with a conspicuous epidermis, usually with concentric 

 fine sculpture ; valves equal, free, closed, with plain margins ; area 

 obscure or none ; with a feeble short ligament, a simple pallia! ine, 

 and no hinge-plate ; cardinal teeth not exceeding 2 in each valve, 

 exhibiting a cessation of development at an early stage ; they are 

 variable, very thin, often nearly parallel to the hinge-margin or de- 

 fective in part of the series ; laterals, anterior and posterior, usually 

 double in the right, single in the left valve, distinctly separated from 

 the cardinals. 



These animals are fluviatile. 



Upper Cretaceous to Recent. 



KEY TO GENERA. 



A. Anterior side of shell a little shorter than the posterior ; 



mantle with 2 posterior siphons . . . . . . SPH^BRIUM. 



B. Anterior side of shell longer than the posterior ; mantle 



with 1 posterior anal siphon . . . . . . CORNEOOYCLAS. 



Genus 1. SPH^ERIUM, Scopoli, 1777. 



Sphceriutu, Scopoli, Intr. ad Hist. Nat., 1777, 397. Type: Tellina cornea, L. 

 Cyclas, Bruguiere, 1798. Cornea, Megerle, 1811. Amesoda, Rafinesque, 

 1820. Cycladites, Kriiger, 1825. Pniim, Bourmii^nat, 18f>4. Corneolu. 

 Clessin, 1890 ; not of Held, 1837. 



Animal with the borders of the mantle smooth ; foot large, lingui- 

 form, very extensile ; siphons moderately long, united at the base, 

 separated into 2 tubes distally, not fringed ; labial palps triangular, 

 lanceolar ; gills unequal, the outer ones shorter. Viviparous. 



Shell small, oval, more or less inflated, subequilateral, with a thin 

 epidermis, smooth or concentrically striated ; beaks slightly inclined 

 forwards : anterior end slightly shorter than the posterior ; margins 

 simple ; 2 cardinal teeth in each valve, the anterior left and pos- 

 terior right cardinals are often bifid ; right valve with 2 pairs of 

 laterals, left valve with a single anterior and posterior lateral tooth ; 

 ligament deep-set, in a groove whose upper edges nearly meet over it. 

 Pallial line simple. 



Distribution. Europe, America. Africa, Asia, &c. ; in rivers and 

 lakes. 



Fossil. Tertiary. 



Vernacular Name. Orb-shell. 



Subgen. 1. SPH,<ERIUM, s. str. 

 Type : 8. corneum, L. 



The nepionic shell passing into the adult without any distinct 

 demarcation ; the anterior end shorter ; ligament subinternal ; the 



