INTIMATE STRUCTURE OF THE SHELLS OF BRACHIOPODA. 27 



Brachiopoda, which are at once understood when the form and arrangement of their 

 component prisms have been duly studied.^ 



The question now arises, — what is the Idstological character of these prisms, and in what 

 way are they formed ? Considering that they bear a certain degree of resemblance, both in 

 form and arrangement, to the component prisms of the shells of Puma, &c. (their chief 

 differences being that they are flattened, instead of being hexagonal or nearly so, and that 

 they lie at a small inclination to the surfaces of the shell, instead of passing vertically from one 

 surface to the other), it might be not unfairly surmised that they are formed, like these, as 

 elongated cells. It seems at first sight an objection to this view, and was long felt as such 

 by myself, that no trace of cellular membrane is discoverable after the action of acid upon 

 the prismatic substance ; but this is true of a great many other shells also, in which, as I 

 think I have shown to be almost certain, if I have not absolutely demonstrated,^ the shelly 

 texture was originally of a truly cellular nature. Now, it is a very curious circumstance, 

 that in the shell of Crania, the greater part of which presents none of the characteristic 

 Brachiopodous structure, there should be one portion, — that to which the muscles (or rather 

 their tendons) are attached, — which is distinctly prismatic, the prisms having very much 

 the appearance of those of Terehratulce, but being arranged vertically to the surface of the 

 shell as in Pinna (PI. V, fig. 8, a, h) : and that in this structure, a series of elongated 

 cells may be distinctly traced, standing parallel to each other, and presenting their 

 prismatic extremities on the inner surface of the shell (PI. V, fig. 8); these retaining their 

 structure and arrangement in the decalcified shell (PI. V, fig. 13). And thus the pre- 

 sumption becomes greatly strengthened, that the component prisms of Terebratulse, 

 Rhynconellae, &c. were originally formed within a cell-membrane, and have taken the shape 

 of the cell-cavity within which they were moulded. Further, the component prisms of the 

 shell of Ehjnconella octoplicata exhibit a remarkable double-oblique striation (PI. V, fig. 6), 

 which somewhat resembles the transverse striation seen on the prisms of Pinna, and which 

 suggests the idea that each of its long prisms is formed by the coalescence of a series of 

 flattened cells linearly arranged, as I have shown to be the case with those of Pinna.^ 



In a large proportion of the species of Brachioj)oda, the shell is traversed by a system 

 of canals, which pass from one surface to the other, for the most part in a nearly vertical 

 direction, and at tolerably regular intervals. The diameters of these canals difier greatly 

 in the several species which exhibit them, as do also their distances from each other, and, 

 consequently, the number which present themselves within a given area ; as will be seen 

 by comparing together figs. 6, 7, 8, II, 12, 14 of Plate IV, and figs. 1, 2, 3 of Plate V, — 



1 The peculiar structure of the shells of Brachiopoda is liable to be much obscured by mounting the 

 sections in Canada balsam, which goes far to obliterate the lines of division between the prisms. It is on 

 this account alone, that the sections delineated in figs. 1 and 2 of PL V do not exhibit the same appearance 

 as fig. 3. 



2 See my Memoir on Shell- Structure, in the ' Report of the British Association for 1847,' p. 98. 



3 Op. cit., 1844, p. 8, and 1847, p. 96. 



