130 DIMYAIRA, 



Genus 29.— PISIDIUM 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 su.iicient difference bet\vixt the animals of the Cyclas and Pisidi. 

 um to warrant the formation of distinct genera ; those of the former having 

 a shorter and tliicker foot in proportion to the size of the auin.al, 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 Pistdium. Plate XV. 

 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.— Zeoc^.f 



Generic C^arac^m— Suborbicular, transverse, equivalve, sub- 



f This genus was instituted by my ingenious friend Dr Leach, and com- 

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

 having been pointed out to him by the Dr. 



