FAMILY CAUDIACEA. 235 



The geographical range of the species is not well determined. It 

 has not been collected in Scotland, and its range on the Continent 

 appears to be confined, so far as habitats are recorded, to the central 

 parts. 



Cycla3 cornea. 



GrENUS II. CYCLAS. 



Animal ; body oval, greyish or dingy white, mantle lobes open in 

 front for the passage of a large, broad, attenuately elongated 

 foot, closed behind to form the siphonal tubes, which are united 

 for a little distance, and then separated, the branchial tube 

 being the longer. 

 Shell ; equivalve, moderately inequilateral, posterior side rather 

 the larger, both sides rounded, sometimes smooth, sometimes 

 concentrically ridged, thin, bluish white, covered by an oHve- 

 horny epidermis, ligament sometimes apparent, sometimes 

 barely discernible, hinge composed of a sublamellar double 

 cardinal tooth, and two elevated lateral teeth in the right 

 valve interlocking with another double tooth, and four lateral 

 teeth, of similar character, in the left valve. 

 We have in Britain five species of Cyclas, of which two, C. cornea 

 and lacustris, are more or less plentiful throughout, the first espe- 

 cially so, in almost endless variety, and both range over Siberia. 

 Of the remaining three, C. pallida and Pisidioides appear in 

 England only, while C. rivicola appears in England chiefly, the 

 only other recorded habitat being the neighbourhood of Dublin. 

 The last-named species, C. rivicola, is one of nmch interest. It is 

 larger than any other Cyclas in any part of the world, and it is a 

 native chiefly of our metropolitan rivers. There are no more Cy- 

 clads in Europe than are to be found in Britain, and only two ad- 

 ditional species have been collected in any other part of our hemi- 

 sphere, one in Borneo of a compressly grooved type, and one in 



