MALACOZOA. TROPIOPODA. LAMELLIBRAXCHIATA. 247 



compressed, the valves meeting at a very acute angle, extremely 

 thin, transparent, greyish-yellow. Gradually the valves be- 

 come more convex, so as to be ventricose in the upper part, 

 and the angle of their meeting at the lower margin becomes 

 less. The umbones are always depressedly convex, and fre- 

 quently capped, having, like many other species of Cyclas or 

 Pisidium, a strongly marked growth-margin separated by a 

 depression from the succeeding part. The colour is always 

 more yellow than grey, sometimes approaching to dull lemon- 

 yellow ; the inner surface white. In old shells, exceeding 

 four-twelfths in length, the convexity is greater toward the 

 umbones, and sometimes considerable toward the inferior mar- 

 gin ; which however never forms more than an angle of 45°, 

 the form is more rhomboidal, the posterior extremity being 

 more truncate, although never decidedly so ; the valves 

 are thicker, and often have an olivaceous tinge ; but still the 

 colour is properly yellowish-grey. Some individuals how- 

 ever are brownish-yellow, some of that colour zoned with paler, 

 and some are of an olivaceous-brown. 



The animal is yellowish-white, or of the same dull pale yel- 

 low as the shell ; but no observations respecting its habits have 

 been made. 



First found by two of my pupils, Mr. Nicol and Mr. Be- 

 veridge, in June, 1842, in the Loch of Skene, ten miles to the 

 west of Aberdeen. Having gone there, on the 2d July, with 

 my class, I found it in the greatest profusion along the shores 

 and on the sandy bottom of the lake, which is about two miles 

 in length, and a mile in breadth, with very few plants grow- 

 ing in it, Lobelia Dortmanna, which however is abundant, 

 being the most conspicuous. Intermixed with the Cyclades 

 were equally numerous shells of Physa fontinalis, and some 

 of a very short-spired variety of Linnaeus pereger. The only 

 other shell found was Planorbis contortus. 



With reference to specimens transmitted by me, Mr. Jenyns 

 sa}*s, " About No. 1. I will not speak positively : I can hardly 

 distinguish it from certain of the many varieties of Cyclas cor- 

 nea, and some specimens approach very nearly to that which 

 I have called stagnicola ; but both that species and cornea call 

 for further examination, in reference to specimens from dif- 

 ferent localities, it being questionable whether some of the sup- 

 posed varieties of each be not true species. The animal and 

 its habits require to be attended to as well as the shell for the 

 purpose of investigating this matter." 



