CHAPTER IV. 



THE CLASSES, ORDERS, FAMILIES, GENERA, SPECIES, AND 

 VARIETIES OF BRITISH LAND AND FRESH-WATER SHELLS. 



(AQUATIC.) 

 CLASS I. MALACOZOA ELATOBRANCHIA. 



SHELL a bivalve, the two valves of which are united along their 

 dorsal margin by a ligament. Body, oval, headless; mantle 

 bi-lobed ; foot tongue-shaped, sometimes provided with a byssus. 

 Respiration performed by gills. 



ORDER I. LAMELLIBRANCHIATA. 



Gills four in number, leaf-shaped, arranged in pairs on each side 

 of the body between the visceral mass and the mantle 



Family I. Sphseriidae. 



Shell equivalve, subglobose ; hinge with lateral and rdinal 

 teeth. Body with one or two siphons at its anterior end. 



i. Sphaerium. Shell nearly equilateral. Mantle with two 

 prominent contracticle siphons. 



(a) Shell yellowish-brown or brownish, finely striated, suborbi- 

 cular ; umbones blunt, nearly central ; ligament small, not visible 

 externally ; muscular impressions faint ; hinge strong with a 

 double cardinal tooth in each valve, and two triangular-shaped 

 teeth in the right valve and four teeth in the left. Length 6 lines ; 

 breadth 4 lines ; thickness 3^ lines. Ditches, marshes, ponds, 

 canals, and rivers. Generally distributed. S. corneum. 1 



1 v. flavescens, paler, not so large, more globular ; v. nucleus., smaller, 

 nearly spherical ; v. Pisidiodes, shell subtriangular, slightly more produced at 

 its posterior slope, ligament just visible externally, transverse striae coarser ; 

 v. minor* shell smaller, nearly globular. 



