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. — Sphaeriidae. 



Shell equivalve, subglobose ; hinge with lateral and cardinal 

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



I. Sphserium. — Shell nearly equilateral. Mantle with two 

 prominent contracticle siphons. 



{a) Shell yellowish-bro .vn or brownish, finely striated, suborbl- 

 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} 



' V. Jlavescens, paler, not so large, more globular ; v. nucleus, smaller, 

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

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

 V. minor, shell smaller, nearly globular. 



