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and obscuring the angular junctions of the cellular structure beneath, 

 as represented at PI. xiv. fig. 7, b b ; but it is often the case that 

 portions of this envelopment have been removed, and the vessels run- 

 ning beneath it are then readily to be seen, as at PI. xiv. fig. 7, a a. 

 Upon tracing these vessels amid the animal remains I obtained by 

 maceration in dilute hydrochloric acid, I frequently found them dis- 

 engaged from the angles, and floating freely and unconformably on 

 the surface of the membrane. This disengagement evidently arose 

 from a rupture of the vascular system, as upon a close examination 

 I observed that the vessels apparent at the natural surfaces, are not 

 the only ones of this description that exist in the shell, but, on the 

 contrary, that there was a series of these vascular networks, em- 

 bracing the elongated prismatic cells, and which, impressing the 

 lines of their courses upon the surfaces, caused the striated ap- 

 pearance that I have before described as existing upon the prismatic 

 cells of Pinna affinis. These layers of vascular tissue are connected 

 with those above and below them by a short vessel which proceeds 

 from each angle of the several meshes of the network, so that the 

 whole forms a regular, connected, and beautiful series, layer above 

 layer, of reticulated vascular structure. When torn asunder in the 

 direction of the natural planes of the shell, the sides of the cells se- 

 parate at the lines where they are embraced by the vessels, and the 

 thin membranes may be seen connecting the layers of vessels in such 

 a manner as to produce an appearance resembling sections of honey- 

 comb, at right angles to the axes of the cells (PI. xiv. fig. 8). I have 

 been unable to detect any diaphragms in the long prismatic cells, at 

 the points embraced by the vascular tissues, by which they might be 

 assimilated in some degree with the tessellated cellular structure of 

 Unio and Mytilus. 



Haversian canals are frequently to be seen, especially immediately 

 beneath the outer surface of the shell. 



The disposition of the cells in the genus Ostrea is widely different 

 from that in Pinna, and in the genera which I have before described ; 

 the whole shell consisting of alternating layers of fleshy membrane 

 and cellular structure, which are produced in succession from the 

 inner surface of the shell. The normal condition of these tissues is 

 exceedingly like that of the young lip of Helix aspersa. 



If we remove the innermost thin laminated portion of the extreme 

 edge of the shell of a half-grown oyster, and examine it immersed in 

 water by transmitted light, with a power of 700 linear, we frequently 

 find the outer margin to consist of a pellucid cartilaginous structure, 



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