ARCA. 



GENERIC CHARACTER. 



Shell transverse, more or less elongated, and oblique i 

 anteriorly subrhomboidal, inequilateral, subequivalve ; 

 summits remote ; hinge rectilinear or slightly curved and 

 furnished with a series of numerous, vertical, inserting 

 teeth, which decrease in size from the extremities to thc 

 centre; two muscular impressions, remote, connected by 

 a simple pallcal line; ligament broad, exteiiding both be- 

 fore and behind the summits, and expanding over iho^ 

 rhomboidal surface between the summits; animal with a 

 byssus. 



OBSERVATIONS. 



Linne included in his genus Area, not only the species 

 of which it is at present constituted, but those also which 

 have been since distiuEfuished bv the names of Pectunculus 

 and Nucula. Gmelin added to it his A. cuculliis, after- 

 wards separated by Lamarck under the generic name of 

 Cucullaea. 



As respects Nucula it appears that the relation to Area 

 is rather one of analogy than of affinity, and it is possible 

 that Turton may be right in separating it entirely from 

 the Arcaceae, with which its numerous teeth bear some 

 resemblance; but the propriety of placing it in the family 



PL 36. 



