THE ANNALS AND MAGAZINE OF NATURAL HISTORY, No. 79. DECEMBER 1843. XL VI. — General results of Microscopic Inquiries into the Minute Structure of the Skeletons of Mollusca, Crustacea and Echino-dermata. By William B. Carpenter, M.D.* [With two Plates.] Having been engaged, during the last eighteen months, in mi-croscopic researches into the skeletons or hard parts of animals, and having had my attention particularly directed towards the shells of Mollusca and Crustacea, and to the shells, spines, stems, &c. of the Echinodermata, I have been led to believe that a short resume of the results at which I have arrived, would be interest-ing to the members of the Geological Section of the British As-sociation, and might prove of importance in giving a stimulus to further inquiry. Were it not, indeed, in the hope that the ad-vance of science might be promoted by my bringing forward these results in their present form, I should have hesitated in doing so, until they had attained more completeness. I shall com-mence with the Mollusca, as presenting the greatest number of points of interest. Mollusca. The first point on which I desire to lay stress, is the complete distinctness of the organic basis of the shells of Mollusca from the calcareous portion. The two are not uniformly diifused through the shell, as some have supposed, but are separate ele-ments, each having its own place. The organic or animal basis presents very definite characters, by which its nature may be de-termined, when the calcareous portion has been removed by the action of an acid. I also think it desirable to state, in limine, that the structure of all shells appears to me to be equally crystalline ; the carbonate of lime uniformly presenting a crystalline arrangement, if not a regular form. I cannot admit, therefore, the distinction between the crystalline and concretionary shells which Mr. Gray has at-tempted to establish ; nor can I recognise the two forms of cry-* Read to the Geological Section of the British Association, August 1843; and communicated by the Author. Ann. ^ Mag. N. Hist. Vol xii. 2 D
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