137 



distinctly seen. In one of these columns, the diameter of which was 

 •j-J-j, three of the spaces between the lines which I measured were 

 T 6 l 6 T , T^Tji ana ttts °f an mcn * n width. When several of the 

 prisms remained attached to each other, the lines on each of the ad- 

 joining columns were observed to correspond with each other, as if 

 they were tessellated cells, not only in columnar arrangement as re- 

 gards each separate prism, but they were also arranged layer above 

 layer, in accordance with the natural planes of the shell ; and this 

 we shall presently find is not their true structure. Upon macerating 

 portions of the recent Pinna in gens, in dilute hydrochloric acid, 

 I obtained the animal matter of the shell in the form of a series of 

 elongated cells, which retained their tubular character when in an 

 undisturbed state, but which readily separated in planes parallel to 

 those of the natural surfaces of the shell. Each of these layers pre- 

 sents such an appearance as a thin slice of the honey- comb of the 

 bee would exhibit, if we were to take a section of it at right angles 

 to the axes of the cells. If the maceration of the piece of shell in 

 the weak solution of acid be arrested before the whole of the earthy 

 matter has been dissolved, there is frequently a portion of carbonate 

 of lime remaining within many of the cells, which illustrates, in a 

 very striking manner, the nature of the structure and contents of these 

 beautiful tissues. The cells have not uniform diameters, but some 

 terminate with a point before they reach the outer or inner surfaces 

 of the shell, while others extend through the whole intervening space. 

 Neither is the number of angles constant, as in a circular space, -j S 

 inch diameter, I found they varied in this respect from three to nine. 

 If a fragment of the thinnest part of the shells of Pinna ingens or 

 saccata be examined by transmitted light, in a little water between 

 glasses, and with the inner surface towards the eye, the mode of ar- 

 rangement of the cells will be displayed in a very satisfactory manner ; 

 and if the power applied be not less than 500 linear, a very beautiful 

 reticulated system of vessels may be frequently observed occupying 

 the angles formed by the terminations of the cells, at the natural sur- 

 face of the shell, as represented at PI. xiv. fig. 6, a. Sometimes a 

 portion of the vascular structure is destroyed, as represented in the 

 figure, and the disrupted ends of the vessels are frequently seen pro- 

 jecting partly across the areas of the network, but the courses of the 

 tissues thus removed may be readily traced by a depressed line, which 

 indicates the directions of their embedments. In other cases the ves- 

 sels are not so readily to be seen, as there is frequently a rude gelati- 

 nous-looking network spread over the vascular structure, filling up 



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