148 DIMYARIA. 



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 re- 

 ceive the opposite ones. 



There is sufficient difference betmxt the animals of the Cyclas and 

 Pisidium to warrant the formation of distinct genera ; those of the 

 former ha\'ing a shorter and thicker foot in proportion to the size of 

 the animal, and having a double crested tube at the posterior termi- 

 nation of the cloak, ^vhereas the latter has only a single tube, which 

 however is composed of two siphons. The characters, therefore, in 

 italics, of the genus Cyclas, are inapplicable to that genus, and hold in 

 tliat of risidium. 



Pisidium ohliquum. — The Oblique Pisidium. 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 external ; umbones usually projecting outwards. 

 The species are all littoral. 



* Tellinaria. 

 a Without lateral teeth. 



Genus 30.— ASTARTE.— ,SoM--er%. 

 CRASSINA.— ZamarcA.f 



Generic Character. — Suborbicular, transverse, equivalve, 



t This genus was first instituted by Sowerby, who named it Astarte. 

 It \vas afterwards characterized by Lamarck, under the name of 

 Crassina. 



