GENERA OF SHELLS. 



CLASS XL — CONCHIFERA. 

 Shell always bivalve, wholly or partly covering 

 the animal; sometimes free, sometimes fixed ; 

 valves mostly joined at the margin by a hinge 

 or ligament ; the shell is sometimes enlarged 

 by testaceous, accessory pieces, not belonging 

 to the valves. 



Order I. — CONCH. BIMUSCULOSA. 



Shell presenting in the interior two separate and lateral 



muscular impressions. 



Section I. — CONCH. CRASSIPEDA. 



Shell gaping at the sides ichen shut. 



TUBICOLARIA. 



Shell either contained in a testaceous tube, distinct from its 



valves, or encrusted, tcholhj or in part, in the parietes of 



this tube, or projecting from it. 



The conchifera of this family are borers, and bury 

 themselves in stones, wood and thick shells, but some 

 remain in the sand. These, as well as the pholades, 

 consist essentially of two similar, equal and regular 

 valves, jointed like a hinge. In consequence of their 

 having accessory pieces, but which do not properly 

 belong to the valves, these shells have been mistaken 

 for multivalves. 



AsPERGiLLUM. Sheath tubular, testaceous, 

 tapering and open anteriorly, and terminated 

 posteriorly by an enlarged mass, presenting at 

 one side the two valves imbedded in its pari- 



