III.] INTRODUCTION. xlv 



of his admirable observations on the land and freshwater 

 Mollusca of France : 



" Les Mollusques ont des ruses et des industries, des 

 sympathies et des inimitie's, des guerres acharne'es et 

 des amours bizarres. Beaucoup sont & la fois male et 

 femelle, et par suite pere et mere .... Malgre* leur 

 apathie apparente, les Mollusques sont des etres qui ne 

 manquent pas d'intelligence. Leur vie prive"e et leur 

 vie commune nous montrent des details extremement 

 curieux." 



CHAPTER III. 



SHELLS. 



FORMATION. COMPOSITION. SHAPE. NUCLEUS. GROWTH. 

 COLOUR. DECOLLATION. EROSION. OPERCULUM. EPIDERMIS. 



Formation. The shells of Mollusca are formed by a 

 secretion from glands of the mantle or cloak. In uni- 

 valves this part of the body only covers the front, and in 

 most cases surrounds the head like a loose collar ; but 

 it is very flexible, and it can be withdrawn or folded 

 back nearly to the top of the spire to repair a fracture 

 of the shell in that part. In bivalves it is double, like 

 the cover of a book. 



Composition. Carbonate of lime is the main ingre- 

 dient ; and the shells of Mollusca differ from the bones 

 of vertebrate animals, as well as from the shells of crabs, 

 sea-eggs, and birds' eggs, in the absence of phosphate of 

 lime. In all these cases, however, the mineral ingre- 

 dients are cemented together by an animal gluten. 

 According to M. Delacroix, the shell of a Helix pomatia 



