56 GENERA OF SHELLS. 



spring from the disc of the small valve, and 

 serve as a support to the animal. 



T. vitrea, dilatata, pisum, globosa, rotundata, flavescens, 

 dentata, dorsata, sanguinea, caputserpentis, truncata, psit- 

 tacea. Fossils numerous. 



LiNGULA. Shell subequivalve, flattened, oval 

 oblong, truncated at summit, somewhat pointed 

 at basC) elevated on a fleshy, tendinous pedicle, 

 fixed to marine bodies. Hinge toothless. 



L. anatina. 



MOLLUSC A. 



Body sometimes naked, either destitute of any solid inter- 

 nal parts, or inclosing a shell or other hard substance, 

 and sometimes provided externally with a shell, cover- 

 ing or sheathing, but never composed of two oppo- 

 site valves, united by a hinge. 



Order L— PTEROPODA. 



Some only, are furnished with a thin, cartilaginous or 

 horny shell. 



HYAL^ANA. 



Hyal^a. Shell horny, transparent, ovate 

 globose, tridentate posteriorly, open at sum- 

 mit and two posterior sides. 



The shell of the hyalœa appears to consist of two 

 valves cemented together. The valves are unequal ; 

 the largest above, rather flattened beneath, the other 

 below, tumid, subglobular, and shortened anteriorly. 

 On each side of the shell is a very open fissure to 

 admit water to the branchiae. 



H. tridentata, cuspidata. 



