114 CYCLADIDiE. 



TclUna stagnicola, Shepp. Linn. Trans- vol. xiv. p. 150. 

 Ci/clas flavescens, Macgilliv. Moll. Aberdeens. p. 246. 



„ citrina. Brown, 111. Conch. G. B. p. 132, pi. 3.9, f. 37 (from type). 



The most frequent, variable, and widely-diffused of our 

 freshwater bivalves is incontestably the Cyclas we are 

 about to describe. The shape is ordinarily suborbicular or 

 rounded-ovate, but is occasionally more produced : the 

 valves are never compressed, and usually are swollen, or at 

 least ventricose ; they are moderately thin, not diaphanous, 

 but little shining, and with their surface, which is never 

 striated in a regular manner, although there are indistinct 

 concentric striulai on many individuals, of an ashy-olive 

 colour, or of a more or less squalid yellow, either uniform 

 or stained above with the former, and below with the latter 

 tint ; occasionally, too, there are yellowish zones on an 

 olivaceous ground, or the shell is altogether of a brownish 

 hue. The diminution in convexity is nearly equal on either 

 side of the umbones ; there is no flattening of the lateral 

 dorsal surfaces. The curve of the ventral margin is gene- 

 rally a little diminished in the centre ; the front dorsal 

 edge is the less elevated, but the more convex and sloping ; 

 the almost horizontal hinder dorsal edge merges, without 

 marked angulation, into the more or less arcuated and little 

 oblique posterior margin, so that both extremities appear 

 more or less rounded, but that of the scarcely shorter an- 

 terior side assumes, from the superior dorsal declination, a 

 more attenuated form. The umbones are broad and obtuse, 

 and do not ordinarily lean to either side ; they are not 

 preceded by any distinct lunular impression, but there is 

 frequently a deceptive dusky stain of colouring matter both 

 there and on the opposite side of the beaks ; the ligament, 

 too, is so small, narrow, and depressed, as almost to be 

 totally imperceptible. 



