246 MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 



The Cycladina inhabit fresh water exclusively, resid- 

 ing in the mud or among the roots of plants. They 

 are scarcely entitled to rank as a family distinct from the 

 Venerina ; but being all inhabitants of fresh water, and 

 having for the most part a peculiar aspect, they may 

 without much impropriety be kept apart. 



GENUS 1. CYCLAS. CYCLE. 



Animal elliptical or subglobose ; with the mantle open 

 anteriorly and beneath, united behind into a tube, inter- 

 nally double, the two siphons separated toward the end 

 and exsertile ; foot tongue-shaped, very extensile. 



Shell elliptical and convex, or subglobose, equivalve, 

 very thin, concentrically striated, with a thin, persistent 

 epidermis. Umbones tumid, obtuse, approximated, 

 little curved, about the middle. Hinge with one tooth 

 in the right, and two in the left valve, besides two la- 

 teral, elongated, lamelliform teeth. 



The species occur in pools, lakes, and rivers, generally 

 inhabiting muddy places. They are viviparous. 



1. Cyclas flavescens. Yellowish Cycle. 



Shell rhomboido-elliptical, ventricose, somewhat inequila- 

 teral; the valves* very thin, semitransparent ; glossy, finely 

 and regularly concentrically striate, with distant, stronger 

 growth-marks ; umbones very obtuse, bulging ; frontal slope 

 about a fifth shorter than dorsal, both convex ; anterior end 

 more rounded and somewhat narrower, posterior slightly an- 

 gulate, or truncato-rotundate ; colour pale greyish-yellow or 

 brown, inside in part white. Length five-twelfths of an inch, 

 height about four-twelfths, thickness three-twelfths. 



The hinge is very thin ; in the right valve are two very 

 small, much compressed oblique teeth, of which the posterior 

 is bifid, and on each side are two elongated pliciform lateral 

 teeth, less elevated than the very thin margin ; in the left 

 valve is a single laminiform tooth, having two denticles, and 

 on each side an elongated lamina, having a deep groove be- 

 tween it and the elevated very thin margin. The ligament 

 although elongated is very thin and inconspicuous. 



Considerable variations in form are presented. Thus, when 

 very young, the shell is broadly elliptical, or ovato-elliptical, 



