130 



Genus 29. — P I S I D I U M Pfeiffer. 



Generic Character. — Animal^, instead of a tubular trachea, with 

 a narrow fleshy 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. 



There is suitlcient difference betwixt tlie animals of the Cyclas and Pisidi- 

 um 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 having 

 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 hold in that of Pisi- 

 dium. 



Pisidium ohliquum. — The Oblique Pisidium. Plate XY. 

 fig. 23. Nearly oval, with unequal sides, ventricose, pellucid ; 

 striated concentrically, having the appearance of ribs under a 

 lens ; yellowish ash-coloured ; inside bluish and not glossy ; 

 beaks somewhat produced, and pointing towards the anterior 

 slope. Length three lines, breadth nearly five lines. Inhabits 

 rivers and streams in Britain. 



Section III. — Tenuipedes. 



The mantle barely united before; foot small, narrow, and 

 compressed ; shell having but a moderate gape. 



TRIBE I. NYMPHACEA. 



Having never more than two primary teeth in the same valve ; 

 shell frequently gaping at the lateral extremities ; ligament ex- 

 ternal*, umbones usually projecting outwards. The species are 

 all littoral. 



* Without lateral teeth. 



Genus 30 CRASSINA.— Zeac/i.f 



Generic C^aracfer. —^Suborbicular, transverse, equivalve, sub- 



+ This genus was instituted by my ingenious friend Dr Leach, and coni- 

 inunieated by him to Lamarck, who adopted it, without acknowledging its 

 having been pointed out to him by the Ur. 



