In the crustacea, the structure of the shell has only been examined in the order of decapods. In this order—in the common crab, for example—the shell consists of three layers, viz., (1) an external horny epidermic membrane covering the exterior; (2) a cellu lar or pigmentary structure; and (d) an internal calcareous or tubular Substance. The horny layer is easily detached'after the shell has been for some time immersed in dilute acid; it is thin and tenacious, and presents no trace of structure. The pigmentary layer is very thin in the crab and lobster, but is much thicker in some other decapods. The internal layer is that which constitutes the chief part of the shell; it Is in this layer that the calcareous matter is chiefly deposited; but even after this has been removed, a very distinct animal basis remains, which closely resembles that which is left after the dentine of the teeth has been deprived of its inorganic constituents.
I For further information on this subject, the reader is referred to Dr. Carpenter's various articles on the microscopic structure of shells, and especially to his article Shell" in the Cyclopoid,;a of Anatomy and Physiology (from which the materials of the resent article have been almost entirely drawn), and to his and its Revelations.
STIELLDRAXE, or SRIRLDRAKE, Tadorne, a genus of ducks of the section having the hind-toe without any pendent membrane. The shelldrakes are a connecting link be tween geese and ducks, having, much resemblance to the former. The species are mostly natives of the southern hemisphere. but the COMNION SITELLDRAKE (T. vulpanser, or bellonii) is common on the sandy sea-shores of Britain; many coming from the n. for the winter, and some remaining all the year, and breeding. making their nests in' rabbit burrows or other•oles in soft soil, whence in some places the shelldralse receives the name of burrow duck. It is a beautiful bird, the sexes nearly alike in plumage; the head and upper part of the neck green, with a collar of white, and a lower collar of rich chestnut, extending over part of the hack, the rest of the back white. The whole length is fully two feet. 'The shelldrake is very capable of being tamed, and breeds in domestication. Its note is a shrill whistle. Its flesh is coarse and unpalatable.—The BUDDY SHELIA:MAKE (7'. rutila), the only other European species, is rare as a British bird, although common in many parts of Europe and Asia.