SHELL OF FRESH-WATER MUSSEL. 127 



much as the economy of the animal requires that it should remain flexible. It is 

 an adaptation of an originally univalve shell. 



The whole shell is generally regarded as a cuticular secretion, the cells near the 

 edge of the mantle forming the epicuticula ; those of the part a little remote, the 

 prismatic layer ; and those of the general surface the nacre. Tullberg appears to 

 take the view that the organic part of the shell is produced by a fibrillation of the 

 cells. F. M tiller believes that it increases by intus-susception, and states that in 

 Anodonta the surface of the mantle is separated from the shell over a large space 

 extending from the pallial line and adductor muscles as far as the attachment of the 

 muscles to the ridge bordering the ligament (supra). Judging from analogy it is 

 more reasonable to class the shell as a cuticular formation. This conclusion is 

 borne out by Osborn's experiments on the Oyster. He studied the formation of 

 shell by snipping away portions of the already formed shell, and placing on the ex- 

 posed surface of the mantle a disc of thin glass such as is used in microscopic 

 work. He found that a gummy film was formed on the surface of the glass by the 

 columnar cells of the mantle, which hardened in forty-eight hours into a tough 

 leathery membrane. Crystals of lime, for the most part scaly, appeared in this 

 membrane, which became stony in six days. It is probable that the gummy films 

 formed by the mantle-cells are charged with lime carbonate, which eventually 

 crystallises, its crystalline form being modified by the organic matter of the films, 

 as is the case with crystalline products formed in the presence of colloid matter. 



The calcareous substance of the shell is chiefly composed of Calcium car- 

 bonate. Traces of Calcium phosphate, silica, alumina are sometimes found. The 

 carbonate of lime is sometimes in the form of Calc-spar, e. g. Ostrea, Pecten, and 

 such shells retain their integrity when fossilised. It is sometimes in the form of 

 Arragonite, in the nacreous layer only, e.g. Pinna, or in the prismatic also, as in 

 a very large number (Sorby). The Arragonite usually dissolves away when the shell 

 is fossilised, and then either the inner layer only or both layers are lost as the 

 case may be, leaving a stony nucleus or cast. 



The thickness of the shell does not depend upon the amount of lime in the 

 waters in which the animal dwells, but rather on the workings of its tissues, 

 modified by surrounding influences, whether chemical or non-chemical. This may 

 be readily seen by a comparison of the dense shell of a Pearl Mussel ( Unto mar- 

 garitifer), from the mountain-streams of Westmoreland, with the thin shell of 

 Anodonta from Oxford waters so much richer in lime. 



Structure of the shell. Ehrenbaum, Z. W. Z. xli. 1884; F. Miiller, Z. A. viii. 

 1885 ; in Anodonta, F. Miiller in Schneider's Zoologische Beitrage, Breslau, i. pt. 3, 

 1885 ; cf. Bronn, Klass. und Ordnungen des Thierreichs, iii. i, p. 330 : and Sorby, 

 Presidential Address, Geological Society's Journal, xxxv. 1879 ; of Cyclas cornea, 

 Leydig, Archiv f. Anat. und Physiol. 1855, and F. M tiller, op. cit. supra. 



Hinge teeth. Neumayr, SB. Akad. Wien. Ixxxviii. Abth. i. 1883. 



Formation of shell in Oyster. Osborn, Biological Studies from the Laboratory 

 of Johns Hopkins University, ii. 1882 ; cf. on Embryo Oyster, Ryder in Bull. U. S. 

 Fish Commission, ii. 1883. p. 383. 



On ' 'Molecular Coalescence ' and on the effect of Colloids upon the form of 

 Inorganic Matter, Ord, Q. J. M. xii. 1872, and St. Thomas's Hospital Reports 

 (N. S.), ii. 1871. 



