83 , 



LaMarck does not mention that the valves are 

 marked with minute and strong longitudinal striae, one 

 valve often overlapping the opposite one, and that the 

 margins of them are crenulated. The different size of 

 the valves has induced collectors to suppose examples 

 of this shell not true pairs; but it is a character pecu- 

 liar to them and some of the G. Area, though not con- 

 stantly observable in all the species. 

 Cucullaea auriculifera. 

 [One fossil species.] 



ARCA. 

 arca no,e. — Linn. 

 Chemnitz 7, tab. 53, Jig. 509- 

 The Arcae, as they are now established by LaMarck, 

 form a numerous and well denned genus, easily known 

 by their general resemblance to the hull of a ship, and 

 they are on a slight examination readily distinguished 

 from their congeners. The G. Arca of Linnaeus is now 

 subdivided into four: — Cucullaea, Arca, Pectunculus, 

 and Nucula; each of which possesses a strong dis- 

 tinctive character, and renders their separation from 

 each other necessary. Shells of the present genus are 

 transverse, subequivalve, inequilateral, apices distant, 

 g2 



