432 STRUCTURE OF SHELLS. 



the shell a foliaceous appearance. In these the calcareous 

 particles are large, opaque, white, and earthy, like chalk. 

 This is well exhibited in the common oyster ; and is also 

 found, not so distinctly developed, in the Pectines, and on 

 the outer surface of those shells which are internally pearly, 

 such as the Haliotides, Turbines, &c. The animal matter 

 between the laminae is sometimes very unequally deposited : 

 it is found forming large brown spots in the pearly coat of 

 many of the Haliotides, especially in the Haliotis midae, 

 and H. splendens, in which these spots produce beautiful 

 variations in the colouring and pearliness of the shell. 



In many of the fresh- water bivalves there is deposited 

 between the layers of the shell a lamina of animal matter, 

 similar to the periostracum. In the genera Etheria and 

 Mulleria, such a coat is deposited between nearly all the 

 layers, giving them a very peculiar olive-green colour, and 

 having minute dots on its surface.* The shells in question 

 appear to be extremely liable to be eroded by the water, 

 and these successive depositions of animal matter enable 

 them to offer a new layer of periostracum to protect each 

 succeeding plate, as the one above it gives way to the 

 destructive influence of the medium in which they reside. 

 A similar deposit of animal matter is also often found form- 

 ing green stains in the pearly inner coat of the various 

 species of Uniones, and it sometimes protects from the action 

 of the water the inner part of the umbones of shells which 

 have been eroded. In the upper valve of Ostrea cornu- 

 copias, I have observed the thick inner layer to be rather 

 prismatic, and the outer part of the laminae to be separated 

 by layers of periostracum. 



In some shells of this kind there are left between the 

 plates larger or smaller spaces, which are generally found 

 filled with water. These spaces are sometimes met with in 

 the common oyster, and they occur not unfrequently in a 

 large Spondylus, which is known to the dealers on this 

 account, by the name of the Water Spondylus. In the latter 

 shell it is not unusual to find these cavities, which are some- 

 times of a large size, in both the valves, recurring one on 

 the top of another, and giving the valve, when cut through, 

 the appearance of a chambered shell ; but having no siphon 

 passing from one septum to the other. There can be little 

 doubt that these laminae, the concave plates at the end of 



* Mr. Goulding observes that the cavity of the Mangrove oyster is often 

 disfigured with calcareous blackened blisters, which are laid by the mantle 

 over any extraneous bodies which happen to intrude within the shell. Zool. 

 Journ. iii. 542. 



