MARINE ANIMALS. 131 



for we find in many, spaces between the layers, sometimes vacant, and sometimes 

 with calcareous matter, which on minute examination proves to be either 

 simple calx, or, in many instances, mingled with minute shells, (chiefly univalves,) 

 sponges, and other minute marine productions. In some of the species, such 

 as the Spnndylus, the spaces between the lamina are very great, and this 

 is conjectured to be a provision to avoid weight, as the shell is large, and 

 nearly three inches in thickness, whilst that of a gigantic Clam, the heaviest 

 known shell, is scarcely one inch, but then the latter is nearly solid, and 

 it is probable, as indeed is partly known, that the animals which are inhabitants 

 of these strong rocks of defence are not very discursive in their habits. 



In some of these there is an apparatus for anchoring the shell to rocks 

 or other substances, (see Fig. 6,) and so hard and firm is this adhesive 



Fig. 6.— Spondylus; a, the adhesive process. 



power, that, in many instances, the stroke of a hatchet only is sufficient to 

 detach them. The material by which they adhere is a species of silky 

 substance which extends from the foot; and this substance is woven into 

 gloves from a kind of Spondylus found at Malta. These are bivalves, and 

 similar in shape to a pear, only more straight and tapering, and the exterior 

 of the shell is covered with open cells, which have unequal, edges, and are 

 the abode of several kinds of minute creatures, the foot protruding from one 

 side at the narrow end of the shell. The common Oyster, when in a state 

 of nature, has the convex shell undermost, and that portion which forms the 

 hinge is of precisely the same structure as the shell itself. We all know 

 very well the beautiful hues which are observable in some shells: now this 

 quality is by no means confined to the Oyster, for a species of Mussel and 

 Cockle found in the China seas present the most delicate and beautiful hues. 

 In my former paper I described how this was produced, and that it was, 

 green only, resident in a membrane, which being of the lightest possible hue 

 in itself, by lying in folds produced the brilliant hues we see, being attended 

 upon by its complimentary colour, crimson, (see Fig. 7.) This structure has 

 been compared by Sir David Brewster to the appearance of planed wood^ 



