252 



CHEMICAL COMPOSITION OF THE SHELL 



2nsana). At the end of several days attachineiit to the cohunella 

 took place, and tlien growth began, the new shell becoming soldered 

 to the old, and the spiral part of the animal Ijeing protected l)j 

 a thin calcareous envelope. A growth of from one to two 

 whorls took place under these conditions. The individuals so 

 treated were always sordid and lethargic, Ijut they bred, and 

 naturally produced a normal asiicrsa offspring.^ In the British 



Fig. 160. — Monstrosities witli two 

 apertures : A, Cylindrella agnesi- 

 ana C. B. Ad., Jamaica ; B, 

 Littorina littorea (from specimens 

 in the British MnseunO. 



Fig. 161. — Cornucopia- 

 .sliaped monstrosity 

 of Helix asjjersa, 

 from Ilfracombe. 

 (British Museum.) 



Museum there is a specimen of one of these artificial unions of a 

 Helix with the shell of a Limnaca stagnalis. 



Composition of the Shell. — The shell is mainly composed 

 of pure carl)onate of lime, with a very slight proportion of phos- 

 phate of lime, and an organic base allied to chitin, known as 

 conchiolin. The proportion of carbonate of lime is known to 

 vary from about 99 p.c. in Stromh/is to alsout 89 p.c. in Turri- 

 tcUa. Nearly 1 p.c. of phosphate of lime has been obtained from 

 the shell of Helix ncmoreilis, and nearly 2 p.c. from that of Ostrea 

 rirginica. The conchiolin forms a sort of memliranous frame- 

 work for the shell ; it soon disappears in dead specimens, leaving 

 the shell much more brittle than it was when alive. Carbonate 

 of magnesia has also been detected, to the extent of •12 p.c. in 

 Telesco2oium and '48 p.c. in Ncptunea cmtifiua. A trace of silica 

 has also occasionally been found. 



^ Cailliaud, Journ. dc Conchyl. vii. p. 231 ; Classie-s, ibid. p. 44. 



