4 228 On a peculiar structure in Shells, 



V — On a peculiar structure in Shells; with some observations on 



the Shell of Spkceruliies. By John Edward Gray, F. R. S-, 



&c. Plate VIII. 



In a paper published in the Philosophical Transactions for the 

 year 1833, I have described three kinds of structure found in such 

 shells as had then come under my observation ; but since that pe- 

 riod Mr G. B. Sowerby has given me an oyster-shell, and Messrs 

 Hudson and Bowerbank have lent me a fossil Sphserulites,* found 

 in the chalk, each of which exhibits a form of structure which I 

 had not before observed, and which may be designated by the name 

 of cellular. 



The shells of this structure appear to increase in size in the same 

 manner as others, — the peculiarity consisting in a deposition of one 

 or more series of reticulations, leaving more or less numerous hol- 

 low polygonal cells between each of the lamina of which the shell 

 is formed. The two shells which exhibit this formation show it in 

 a very different state and degree of developement. In the Sphasru- 

 lites the entire parietes of the shell, (or at least the whole that is 

 left in a fossil state, for some naturalists, as M. Deshayes and Des- 

 moulin, believe that, from the form of the internal cast, the inner 

 part of the shell is deficient,) are formed of series of continuous 

 longitudinal and transverse ridges, leaving four-sided cavities, which 

 are hollow in the specimens preserved in chalk, while in those that 

 are found in limestone, they are filled up with infiltrated carbonate 

 of lime. The concentric or transverse plates, which are best seen 

 in a longitudinal section of the valves, and which represent the la- 

 minae of growth, though remarkably regular in appearance, vary in 

 the distance they are apart from each other. They are usually 

 much closer together at the lips of the valves, or, in other words, 

 when the animal has nearly reached its full growth ; but sometimes 

 we find them almost equally near in the middle of the cone, which 

 may have been occasioned by some accidental check to the mollusc's 

 regular increase about that period, and which removed or overcome 

 again admitted the animal to progress at its ordinary rate. 



* This appears to be the fossil which Mr Mantell has indicated, but not de- 

 scribed, under the name of Hippurites Mortonii. I say appears, for on going 

 to Brighton to examine his specimen, I could not obtain permission to have it 

 taken from the case to compare it with that here described. It is certainly not 

 a Hippurites, since it has neither the solid structure, nor the two internal longi- 

 tudinal ribs of that genus. It is the shell figured as a fossil Conia by Mr Hud- 

 son in Loudon's Magazine of Natural History, Vol. ix. p. 103. 



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