418 EXTERNAL FORM OF SHELLS. 



to the plane surface of mother-of-pearl shells have always 

 a flat side, whilst those which are adherent to coral and to 

 other uneven surfaces are variously and irregularly shaped. 

 The oysters which are attached to the branches of mangrove 

 trees have a central convex rib, modelled on the shape of 

 the branch, from which the plaits of the shell radiate ; while 

 the specimens of the same species fixed to the trunk are 

 destitute of any such peculiarity. In the collection of 

 Mr. Adamson, of Newcastle, there is a curious specimen of 

 a common oyster found in the Frith of Forth attached to 

 a species of Pecten ; on the latter there also grew three coral- 

 lines, surrounding the oyster, which have formed in its cir- 

 cumference three deep notches, giving it the form of an ace 

 of clubs. The same changes may be observed to take place 

 in the Anomise, which are attached by the intervention of a 

 ligamentous band ; thus the species called Anomia squa- 

 mula is founded on young shells of the common A. ephip- 

 pium attached to flat surfaces ; w^hilst A. cymbiformis is 

 characterized from small specimens of the same shell attached 

 to the cylindrical spines of sea eggs, or to the stems of sea- 

 weeds. 



A similar influence is also observable in such free uni- 

 valve shells as have a widely expanded aperture, and an ani- 

 mal provided with a large foot, by means of which they 

 remain for a considerable length of time adherent in the 

 same situation. When a Patella or a Crepidula has at- 

 tached itself to the flat surface of a rock or the leaf of a 

 large Fucus, the base of its shell is flat, and its mouth 

 roundish ; when it adheres to a concave surface, such as the 

 cavity of an old shell, the base becomes flattened, and con- 

 vex internally ; and when it flxes itself on the rounded 

 stalk of a Fucus, the sides become compressed so as in 

 some measure to clasp the stem, and the lateral portions 

 of the base project beyond the front and hinder parts, to 

 such an extent that when placed on a flat surface it rocks 

 backwards and forwards. Several nominal species of these 

 and allied genera depend on variations in the shape of the 

 shell caused by the adhesion of the animal to surfaces of 

 different forms ; thus the Patella pellucida of Montagu is 

 synonymous with the P. caerulea of the same author, the 



and their colour, most probably, by the food and by tlieir greater or less 

 exposure to light. The Chania that has lived in deep and placid water will 

 generally be found with its foliations in the highest degree of luxnriancy, 

 while those of the individual which has borne the buffeting of a compara- 

 tively shallow and turbulent sea will be poor and stunted." — Zool. Tram. 

 Lond. i. 301. 



