IX 



GROWTH OF THE SHELL 



257 



the main, protective, and secures to its owners immunity from 

 the attacks of predatory fishes. 



'Attached' genera (e.g. Chama^ Spondylus') when living on 

 smooth surfaces have a flat shell, but when affixed to coral and 

 other uneven surfaces they become very irregular in shape. The 

 sculpture of the base on which they rest is often reproduced in 

 these ' attached ' shells, not only on the lower, but also on the 

 upper valve, the growing edge of which rests on the uneven 

 surface of the base. Oysters attached to the branches of the 

 mangrove frequently display a central convex rib, modelled on 

 the shape of the branch, from which the plaits of sculpture radi- 

 ate, while specimens fixed to the smooth trunk have no such rib. 

 Crepidula^ a genus which is in the habit of attaching itself to 

 other shells, varies in sculpture according to that of its host. 

 Sometimes the fact may be detected 

 that a specimen has lived on a 

 ribbed shell when young, and on 

 a smooth one when old, or vice 

 versa. A new genus was actually 

 founded by Brown for a Capidus 

 which had acquired ribs through 

 adhesion to a Pecten. A specimen 

 of Hinnites giganteiis in the British 

 Museum must at one period of its 

 growth have adhered to a surface on fig. I66. — A specimen of Anomia 



which was a Serpula, the impression ephipplum L., Weymouth, taken 



of y.'hich is plainly reproduced on 

 the upper valve of the Hinnites} 



Growth of the Shell. — Noth- 

 ing very definite is known with 



regard to the rate of growth of the shell in marine Mollusca. 

 Under favourable conditions, however, certain species are known 

 to increase very rapidly, especially if the food supply be abun- 

 dant, and if there is no inconvenient crowding of individuals. 

 Petit de la Saussaye mentions^ the case of a ship which sailed 

 from Marseilles for the west coast of Africa, after being fitted 

 with an entirely new bottom. On arriving at its destination, 

 the vessel spent 68 days in the Gambia River, and took ^Q days 

 on its homeward voyao^e. On beingf cleaned immediately on its 

 1 J. E. Gray, Phil. Trans. 1833, p. 77-4 f. "^ Journ. de Conchyl. iv. p. 424. 



VOL. Ill s 



upon Pecten maxiynus, the sculp- 

 ture of which is reproduced on 

 the upper valve of the Anomia, 

 and even on a young Anomia 

 attached to the larger specimen. 



