The various details of sculpture on the exterior surface of the shell, the striae, ribs, nodules, imbrications, spines, and other forms of ornamentation are all the product of similar and corresponding irregularities in the mantle margin, and have all been originally situated at the edge of the lip. Spines, _e.g._ those of _Murex_ and _Pteroceras_, are first formed as a hollow thorn, cleft down its lower side, and are afterwards filled in with solid matter as the mantle edge withdraws. What purpose is served by the extreme elaboration of these spiny processes in some cases, can hardly be considered as satisfactorily ascertained. Possibly they are a form of sculptural development which is, in 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 radiate, 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 versâ_. A new genus was actually founded by Brown for a _Capulus_ which had acquired ribs through adhesion to a _Pecten_. A specimen of _Hinnites giganteus_ in the British Museum must at one period of its growth have adhered to a surface on which was a Serpula, the impression of which is plainly reproduced on the upper valve of the _Hinnites_.[339] [Illustration: FIG. 166.--A specimen of _Anomia ephippium_ L., Weymouth, taken upon _Pecten maximus_, the sculpture of which is reproduced on the upper valve of the _Anomia_, and even on a young _Anomia_ attached to the larger specimen.]