248 On the Form, Growth, and Construction of Shells. 



happy to gratify any of the readers of this article with a view 

 of his collection, which is well worthy of inspection. There 

 are also several cases for the exhibition of abnormal shells in 

 the gallery of the British Museum. 



Fig. 9. The Common Whelk, Buccinum undatum (British) . 



The Epidermis. — The exterior of the shell of the whelk (see 

 Woodcuts, Figs. 7 — 9) is invested with a thin, straw-coloured 

 membrane whose existence is scarcely recognized. Shells from 

 a quiet soft sea-bed often have a coat like brown velvet. Many 

 exotic Tritons are remarkable for their rough cuticle. Arctic 

 shells, also, such as Modiola, Astarte, Cyprina, have, when 

 taken alive, a very dark-brown epidermis, entirely covering 

 their surfaces. Fresh- water mussels and melanias (see Wood- 

 cuts, Figs. 10 — 12) are also conspicuous by their olive or 

 chesnut-coloured skin. 



FiG. 10. Dreissena polymorpha, 

 showing foot, byssus (b), and 

 respiratory siphons. 



Fig. 11. Three Dreissenas 

 attached to Caspian 

 Cockle. 



This living membrane extends beyond the margins of the 

 valves in a fringe or sheath, and protects the shell from decom- 

 position by chemical action.* The mollusca inhabiting fresh 

 water arc especially exposed to corrosive action, either from 



* In fact, it fulfils exactly a similar office to the human scarf-skin, having life 

 without sensation, it protects the shell whilst the animal lives. 



