377 



VENERID.^. 



THE VENUS TRIBE. 



Most of the tribes, of which we have hitherto described 

 British representatives, are such as severally included 

 forms of shell very different, with animals strikingly simi- 

 lar. In that now before us, the aspect of the shells, 

 though variable, always indicates their natural affinity 

 with each other, whilst many characters in their animals, 

 hitherto of great sectional value, become variable, and, 

 at most, generic. The shells of the Venerida present 

 numerous modifications of colour and sculpture, and are 

 often of considerable thickness ; their hinge is always con- 

 siderably developed, and the teeth strongly marked and 

 distinct ; their margin is in some plain, in others crenu- 

 lated, indicating differences, usually of generic value, in 

 the structure of the mantle ; the ligament is external ; 

 the muscular impressions are always strongly marked, and 

 the pallial has a conspicuous sinus ; the animal has its 

 mantle rather freely open in front for the passage of a 

 large and thick foot ; the siphons are united or separate, 

 according to the genus, and have their margins either 

 fringed or almost plain. The species of the several divi- 

 sions have very different habits of life. 



Dr. Carpenter's researches shew that the shells of this 

 tribe present little organic structure, and are among the 

 hardest of bivalves, approaching the porcellanous univalves 



VOL. I. 3 c 



