378 Dr. W. B. Carpenter on the Minute Structure of 



stallization, the rhomhoidal and prismatic, as characterizing dif- 

 ferent groups, for reasons which will appear in the course of this 

 paper. The crystalline aggregation of the particles of calcareous 

 matter in shell appears to me to be proved by the transparency 

 of thin sections (except where patches of uncrystallized or chalky 

 matter sometimes present themselves), and by their effects upon 

 polarized light. It is very evident in some few instances, in which 

 a distinct crystalline form is presented. One of the most remark- 

 able of these is the tooth of My a arenariu, a section of which pre- 

 sents the remarkable appearance shown in fig. 8 (Plate XIV.), 

 bearing a strong resemblance to that of Wavellite or radiating 

 Arragonite. I have found distinct rhomhoidal crystals in certain 

 imperfectly calcified layers, which occasionally present themselves 

 in Ostrea edulis; and similar crystalline deposits have been 

 described as existing in the dense integument of some Ascidians. 

 The first distinction established by the microscope, between the 

 different forms of shell- structure, has reference to the nature of 

 their organic basis. This, in most shells, consists but of a sim- 

 ple membrane, of extreme tenuity, and not presenting the slightest 

 trace of structure, even when examined under the highest mag- 

 nifying powers. This membrane is obviously analogous to the 

 basement membrane lately described by Mr. Bowman (^ Cyclopae- 

 dia of Anatomy and Physiology,' art. Mucous Membrane) as ex- 

 isting between what is commonly termed the true skin and the 

 overlying epidermis, and also between the ordinary mucous mem- 

 brane and its epitheliura ; and I am inclined to believe that it 

 originally holds the same position on the mantle of the mollusk, 

 and that it is cast off with each new layer of shell. We shall pre- 

 sently have to notice the remarkable variations, in the microscopic 

 appearance of sections of shell, which are produced by the dif- 

 ferences in the manner in which this membrane is arranged. It 

 may be easily obtained by submitting the internal layer of any 

 shell (whether nacreous or not) to the action of dilute muriatic 

 acid, until all the calcareous matter has been removed. In the 

 porcellanous shells it is so extremely delicate, that, when thus se- 

 parated, it is liable to be overlooked ; and it seems to be dissolved 

 by the acid, if left in it for too long a time, or if the acid be too 

 strong. 



In certain bivalve shells, however, an entirely different kind of 

 membranous basis remains behind, after the calcareous matter has 

 been removed by acid. This consists of a stratum of prismatic cells, 

 adherent together at their sides, and forming by their extremi- 

 ties the two surfaces of the membrane, which thus presents the 

 appearance of a honey-comb. This is most characteristically 

 seen in Pinna ; nearly the whole of which shell is formed of this 

 prismatic cellular substance, as may be easily shown by examining 



