398 TEGUMENTARY ORGANS 



rows, in the prismatic structure, but bursting and becoming confused 

 into a homogeneous tissue, in the membranous substance. Nor,, 

 indeed, would it have been very easy in 1 848 to arrive at any other 

 conclusion than this, to which so great a number of appearances at 

 first sight tend. Enabled, however, by Dr. Carpenter's great kind- 

 ness and liberality to form my own judgment from his beautiful 

 preparations, and having also worked over the fresh shells for myself,, 

 I have come to very different conclusions. I will not say that 

 occasionally cells may not be enclosed in shell, but I believe I am in 

 a position to show that, as a rule, shell-growth is not a case of 

 conversion, but one of excretion, cells not being in any way directly 

 concerned in the matter. 



We may consider, first, the growth of the shell as a whole ; and,, 

 secondly, that of its three constituents. Inasmuch as we know, that 

 the shell of the young Unio or Anodon was once as thin as, or thinner 

 than, the " epidermis " of the adult shell, and smaller than the 

 smallest area, bounded by a concentric line on its outer surface ; 

 further, since we know that no addition is made to the outer surface 

 of the shell directly ; it is clear that the shell must grow in size by 

 addition to its margin ; in thickness, by addition to its under surface. 

 Furthermore, since the extreme margin of any shell is constituted by 

 the horny " epiderm," internal to which is the gradually thickening 

 layer of prismatic substance, constituting the brown zone within which 

 again is the white nacreous area, formed by the superposition of mem- 

 branous layers over the fully-formed thick prismatic substance ; from 

 all this it appears to be equally certain that any given spot of the 

 mantle of a young bivalve must give origin, directly or indirectly,, 

 first, to ■' epiderm " ; secondly, to prismatic substance ; and, thirdly,, 

 to nacreous substance ; so that, on examining the free edge of a 

 growing shell, we ought, since the " epiderm " is structureless and 

 transparent, to be able to observe the gradual formation of the- 

 prismatic substance upon its under surface. This is, in fact, the case. 

 Fig. 3 1 3, A, represents such a free edge of the shell of Anodon, a being 

 the direction of the flexible zone ; b, that of the perfect prismatic 

 substance. 



Dr. Carpenter describes the appearances here figured in the follow- 

 ing terms (/. c. p. 8.) : 



"Although the prismatic cellular structure has not yet been 

 actually observed in process of formation, yet certain appearances, 

 which are occasionally met with in the marginal portions of its newest 

 layers, throw great light upon its mode of growth, and indicate its 

 strong resemblance to cartilage in this respect ; for in these situations 



