444 cyprinidjE. 



short, the branchial wide, compressed, the orifice ciliated ; 

 the anal narrower, conical, with the apex crowned with short 

 cilia; gills large, quadrangular, unequal, united posteriorly. 

 Foot thick and linguiform. 



Shell regular, equivalve, sub-cordiform, covered with a 

 thick, dark, shining epidermis. Hinge with three, unequal, 

 divaricate teeth in each valve, and a lateral tooth, remote 

 from the hinge, on the hinder side; ligament external, con- 

 spicuous. 



Genus CYPRINA, Lamarck. 



Shell more or less orbicular, solid, equivalve, closed, 

 covered with a horny epidermis ; beaks prominent, in- 

 curved ; lunule none ; margins smooth. Hinge with three 

 primary teeth, and one lateral tooth in each valve. Pallial 

 impression simple, or with a very slight siphonal inflection. 



Si/ /i. Cardia, Olafsen, not Hill. Cyclas, Link, not 

 Klein or Brug. Arctica, Schum. Asmidia, Gistel. 



E.v. C. Islandica, Linnaus, pi. 110, fig. 4. Shell, 

 C. Islandica, fig. 4, a, 4, b. 



The hinge teeth are 3 8, triangular ; the front one of 

 the left valve is conical and rugose, like the anterior lateral 

 tooth of Venus, and the hinder one is very thin and 

 compressed ; the middle one of the right valve is com- 

 pressed, and the hinder one is very broad, with a deep 

 groove. As Dr. Gray has observed, the shells of this genus 

 have very much the appearance of those of Astarte, but 

 the teeth and form are more like those of the Veneridce. 

 The only species known is from the seas of Europe. 



