252 



CHEMICAL COMPOSITION OF THE SHELL 



pisana). At the end of several days attachment to the colu- 

 mella took place, and then growth began, the new shell becom- 

 ing soldered to the old, and the spiral part of the animal being' 

 protected by a thin calcareous envelope. A growth of from 

 one to two whorls took place under these conditions. The indi- 

 viduals so treated were always sordid and lethargic, but they 

 bred, and naturally produced a normal aspersa offspring.^ In 



Fig. 160. — Monstrosities with two 

 apertures: A, Cylindrella ag- 

 nesiana C. B. Ad., Jamaica; B, 

 Littorina littoi^ea (from specimens 

 in the British Museum) . 



Fig. 161.— Cornucopia- 

 shaped monstrosity 

 of Helix asperfia, 

 from Ilfracombe. 

 (British Museum.) 



( 



the British Museum there is a specimen of one of these artificial 

 unions of a Helix with the shell of a Limnaea stagnalis- 



Composition of the Shell. — The shell is mainly composed 

 of pure carbonate 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 Strombus to about 89 p.c. in Turri- 

 tella. Nearly 1 p.c. of phosphate of lime has been obtained 

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

 Ostrea virginica. The conchiolin forms a sort of membranous 

 framework 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 Telescopiitm and -48 p.c. in Neptunea antiqua. A 

 trace of silica has also occasionally been found. 



1 Cailliaud, Journ. de Conchyl. vii. p. 231; Gassies, ibid. p. 44. 



