394 DR. CARPENTER'S RESEARCHES : 



the shells of molluscous animals possess organic structure, 

 but he explains differently the appearances indicative of 

 their vascularity, and denies their vascular connection with 

 the animal. He bears witness to the correctness of Mr. 

 Bowerbank's description of the structure of the univalves ; 

 but he discriminates with nicer precision the various struc- 

 tures exhibited by the bivalves. Of these beautiful organ- 

 isms I could not give you an intelligible notion without the 

 aid of the figures which illustrate Dr. Carpenter's Reports, 

 and the attempt would be beside my purpose in this place, 

 where I wish only to indicate the principal varieties — which 

 are named the Cellular, the Membranous, and the Can- 

 cellated. 



Of the cellular variety, the Pinna affords the most charac- 

 teristic example. The shell is composed of a vast multitude 

 of prisms, having for the most part a tolerably regular hex- 

 agonal shape and nearly uniform size. These are arranged 

 perpendicularly, or nearly so, to the surface of each lamina, 

 so that the thickness is formed by their length, and the two 

 surfaces by their extremities. A " satisfactory view of 

 these prisms is obtained by grinding down a lamina until it 

 possesses a high degree of transparency, and it is then seen 

 that the prisms themselves appear to be composed of a very 

 homogeneous substance, but that they are separated by defi- 

 nite and strongly-marked lines of division. In general the 

 substance forming the prisms is very transparent, but here 

 and there is seen an isolated prism, visually of smaller size 

 than the rest, which presents a very dark appearance, even 

 in a section of no more than 1- 400th of an inch in thickness, 

 as if the prism contained an opaque substance." This opa- 

 city appears to be due to the presence of a small quantity of 

 air in or near the extremities of the cells. 



The membranous structure is the prevalent one, embracing 

 all those shells that do not present the prismatic cellular 

 tissue. In these the calcareous matter is deposited in 

 laminae separated by an excessively thin membrane, which 

 forms, in fact, a secreting surface. No trace of cells 

 can, for the most part, be discovered ; and when they do 

 present themselves, they are usually scattered through the 

 substance with little or no regularity, and do not form a 

 continuous stratum when the calcareous matter has been 

 removed by an acid. " In no shell," says Dr. Carpenter, 

 " even those most decidedly porcellanous, have I failed in 

 detecting some membranous basis, although the film is often 

 of extreme tenuity. I believe that there is no shell, in 

 which this kind of structure does not exist under some 



