CONCHIFERA. 
452 
This genus has two sections.—* With the lateral teeth crenu- 
lated or serrated.—** Having the lateral teeth entire. 
Genus 28. — CYCLAS_ Lamarck. 
Generic Character. — Ovate, globular, transverse, and equi- 
valve, with the beaks tumid; cardinal teeth very small, some¬ 
times barely perceptible; or with two in each valve, of which one 
is complicated; or only one plicated or lobed tooth in one valve, 
and two in the other; lateral teeth transversely elongated, com¬ 
pressed, and lamelliform ; ligament external. 
Cyclas cornea. — The Horny Cyclas. 
Elate XC. fig. 2. 
Suborbicular, convex, thin, pellucid, with fine concentric striae; 
covered with a horn-coloured epidermis; bluish white within. 
Three-fourths of an inch long. Inhabits rivers of Europe. 
Genus 29 _PI S ID IUM. —Pfeiffer. 
Generic Character. —Animal, instead of a tubular trachea, with 
a narrow liesby projection next the fore part of the shell; the 
foot long and thin ; shell equivalve, transverse, sides unequal, 
completely closing ; in the right valve one, and in the left valve 
two opposite, very small primary teeth; behind and before, two 
thin lamellar side teeth ; those of the latter cleft in the right 
valve, in order to receive the opposite ones.* 
Pisidium ohliquum. — The Oblique Pisidium. 
Plate LXXXVIII. fig. 23. 
Nearly oval, with unequal sides, ventricose, pellucid; striated 
concentrically, having the appearance of ribs under a lens; yel¬ 
lowish ash-coloured ; inside bluish and not glossy; beaks some¬ 
what produced, and pointing towards the anterior slope. Length 
three lines, breadth nearly five lines. Inhabits rivers and streams 
in Britain. 
• There is sufficient difference betwixt the animals of the Cyclas and 
Pisidium to warrant the formation of distinct genera; those of the former 
having a shorter and thicker foot in proportion to the size of the animal, 
and with a .double crested tube at the posterior termination of the cloak, 
whereas the foot of the latter is a single tube. The characters, therefore, in 
italics, of the genus Cyclas, are inapplicable to that genus, and obtain in 
that of Pisidium. 
