10 REPORT 1844. 



number of minute molecules. In other cases, again, I have found it studded 

 here and there with what seemed to be incipient cells. And lastly, I have 

 occasionally found these cells more developed, and forming an almost conti- 

 nuous layer on the surface of the membrane. In this state they somewhat 

 resemble the incipient form of the prismatic cellular substance. These cells 

 may be occasionally seen in sections of the shell itself ; and they will be often 

 found in very different degrees of development, even in the corresponding 

 layers of two shells of the same species. Coupling the appearances which I 

 have myself observed with the observations of Mr. Bowerbank on the forma- 

 tion of shell, and keeping in view the general doctrines of cell-action, which 

 I have elsewhere endeavoured to develope, I am inclined to believe that 

 these cells are, like the cells of the prismatic cellular structure, the real 

 agents in the production of the shell, it being their office to secrete into their 

 own cavities the carbonate of lime supplied by the fluids of the animal. 

 But whilst the cells of the prismatic cellular structure advance in their de- 

 velopment, so as to form a perfect tissue, — the "calcigerous cells," of which we 

 are now speaking, appear to burst or liquefy, and to discharge their contents 

 upon the surface of the subjacent membrane, on which a shelly layer is 

 thus formed. A greater or smaller proportion of these being left entire, 

 and being included in the substance discharged from the rest, would pre- 

 sent the appearances I have mentioned as occasionally manifesting them- 

 selves in sections of membranous shell-structui'e, and in the decalcified mem- 

 brane. Thus in Mi/a, Anatina, Thracia, and other allied genera, I have met 

 with obvious indications of a cellular structure in sections of the exterior 

 layer of the shell (fig. 15) ; but I have seldom been able to obtain any distinct 

 layer of cell-membrane (like that existing in the shell of Pinna and its allies) by 

 the action of acid, except in Thracia and Pandora ; although traces of scattei-ed 

 cells do present themselves. Hence it is evident that the cells, if they ever 

 existed as such (of which I have little doubt), have ceased to exist ; but that 

 their solid contents have been left. The difference between this kind of 

 structure and the regular prismatic cellular substance, will be made evident 

 by a comparison of the two forms delineated in figs. 3 and 15. The sharp- 

 ness and definiteness of the lines dividing the cells in the former, are in 

 striking contrast with the irregularity of the spaces intervening between the 

 latter. In the shells of the family Myidee, too, I have seen other appearances 

 wjiich fall in with the view just expressed in regard to the "fusion" of cells 

 with each other ; these I shall describe more particularly in a future Report ; 

 but in the mean time I may direct attention to fig. 13, as most clearly indi- 

 cating the existence of such a "fusion ;" its various stages being evident in 

 the different parts of the same specimen. 



18. Tiie Membranous shell-substance presents many curious varieties of 

 aspect, which may be generally accounted for by corresponding diversities in 

 the arrangement of the basement-membrane. Thus it sometimes presents a 

 simple homogeneous character, as if the shelly matter had been uniformly 

 diffused over a plane surface ; but this is comparatively seldom the case, for 

 there are few instances in which the shell does not present, in some part of 

 its thickness, an appearance which indicates an unevenness of surface on the 

 jiart of the basement-membrane (fig. 43) ; and this appearance is usually 

 found to correspond with the aspect of the membi-ane after decalcification. 

 Sometimes this unevenness amounts simply to a corrugation or wrinkling, 

 closely resembling that of morocco leather. The boundaries of the wrinkles 

 are so strongly marked in some shells, that even the experienced Microscopist 

 may be deceived into the belief that he is looking at a section displaying fusi- 

 form cells. Such is the case with the inner layer of the shell of Patella. In 



