CKOWTII Ol" 'I'll I-; SIIKLL 



257 



tective, and secures to its owners immunity from the jittacks of 

 predatory fishes. 



' Attached ' genera (e.g. Cliama, Sj)07idylus) when living on 

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

 other uneven surfaces they become very irregular in shape. The 

 sculpture of the base on which they ro!st is often r('])ro(hu'(Ml 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 li'(M[uently display a central convex rib, modelled on the 

 shape of the l)ranch, from which the plaits of sculpture radiate, 

 while specimens fixed to tlie smooth trunk \\\\\v ii(» such ril). 

 Crcpldala, a genus which is in the habit of attaching itself to 

 otlier shells, varies in scidpture according to that of its host. 

 Sometimes tlie I'act may be detected 

 that a specimen has lived on a 

 riV)bed shell when young, and on 

 a smooth one when old, or vice. 

 versd. A new genus was actually 

 founded by Brown i'or a, Cajmlus 

 which had acijuired ril)S througli 

 adhesion to a Fecten. A specimen 

 of Hinnites giganteus in the Britisli 

 MuseuuL must at one ])eriod of its 

 growth have adhered to a surface on 

 which was a Serpula, the impression 

 of wliich is ])lainly reproduced on 

 the up]M'i- valve of tiie Himiites} 



Growth of the Shell. — No- 

 thing very definite is known with 

 reward to the rate of urowth of the shell in marine Mollusca. 

 Under favourable conditions, however, certain species are known 

 to increase very rapidly, especially if the food supply be aljundant, 

 and if there is no inconvenient crowding of individuals. Petit 

 de la Saussaye mentions- the case of a shij) which Siiiled from 

 Marseilles for the west coast of Africa, after being fitted with an 

 entirely new l)ottom. On arriving at its destination, the vessel 

 spent G8 days in the (Jand)ia IJivei-, and took 80 days on its 

 homeward voyage. On being cleaned innuediately on its return 



1 J. E. Gray, Phil. Trans. 1833, \\ 774 f. - Journ. de ConchijI. iv. p. 421. 



VOL. Ill S 



Fig. 160. — A specimen of Anomia 

 ephi^ypiinn L., Weyinoutli, taken 

 upon Pecten maximus, the sculp- 

 ture of which is reproduced on 

 the upper valve of the Anoviin, 

 and even on a young Anomia 

 attached to the larger specimen. 



