596 PHILOSOPHY OF ZOOLOGY. 



while the depressed part, of the same form, has the base at 

 the mouth. The former consists of conical vertical tubuli, 

 while the latter appears solid. When the base is testa- 

 ceous, it is either solid, or consists of horizontal tu- 

 buli, radiating from a centre united by a simple layer, ex- 

 hibiting concentric circles. 



The structure of the valves gives sufficient indications 

 of the manner in which they have been formed. M. Du- 

 FRESNE, in a paper published in the Annales du Museum, 

 vol. i. p. 465, advanced the singular opinion, that the animals 

 quitted their old shells when they became toosmall, and form- 

 ed new ones suited to their size. The arguments by which 

 it is supported, indicate inattention to the structure of the 

 shell, and the relation of the parts of which it consists. To 

 us it appears obvious, that each valve is increased in two 

 directions, the elevated part by an extension of the tubuli 

 at the base, and the depressed part, by the application of 

 fresh matter to the side. ' The striae, which are the indica- 

 tions of successive depositions of matter, and the structure 

 of the valves themselves, point out this mode of enlarge- 

 ment as the only one which can take place, even on the 

 supposition that the shell is frequently renewed. By the 

 growth of the elevated parts, the shell increases in height 

 and diameter at the base, while the growth of the depres- 

 sed parts, preserves to the mouth suitable dimensions for 

 the corresponding increase of size in the parts of the oper- 

 culum. It is obvious that this increase of diameter at the 

 base must be accompanied by a corresponding enlargement 

 of its covering. This takes place by the extension of the 

 horizontal tubuli, and each enlargement is marked by a con- 

 centric ridge. 



This opinion here advanced, and which we find indis- 

 tinctly hinted at by LAMARK, in his " Histoire Naturelle 

 des Animaux sans Vertebres" vol. v. p. 389, is founded on 

 ihe structure of the different valves, the indications of the lay- 



