Some decades ago ingenious Frenchmen amused themselves by creating artificial monstrosities. _H. aspersa_ was taken from its shell, by carefully breaking it away, and then introduced into another shell of similar size (_H. nemoralis_, _vermiculata_, or _pisana_). At the end of several days attachment to the columella took place, and then growth began, the new shell becoming 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 individuals so treated were always sordid and lethargic, but they bred, and naturally produced a normal _aspersa_ offspring.[334] In the British Museum there is a specimen of one of these artificial unions of a _Helix_ with the shell of a _Limnaea stagnalis_. [Illustration: FIG. 160.--Monstrosities with two apertures: =A=, _Cylindrella agnesiana_ C. B. Ad., Jamaica; =B=, _Littorina littorea_ (from specimens in the British Museum).] [Illustration: FIG. 161.--Cornucopia-shaped monstrosity of _Helix aspersa_, from Ilfracombe. (British Museum.)] =Composition of the Shell.=--The shell is mainly composed of pure carbonate of lime, with a very slight proportion of phosphate 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 _Turritella_. 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 _Telescopium_ and ·48 p.c. in _Neptunea antiqua_. A trace of silica has also occasionally been found. When the shell exhibits a crystalline formation, the carbonate of lime may take the form either of _calcite_ or _aragonite_. The calcite crystals are rhombohedral, optically uniaxal, and cleave easily, while the aragonite cleave badly, belong to the rhombic system, and are harder and denser, and optically biaxal. Both classes of crystal may occur in the same shell. Two main views have been held with regard to the formation and structure of the shell--(1) that of Bowerbank and Carpenter, that the shell is an organic formation, growing by interstitial deposit, in the same manner as the teeth and bones of the higher animals; (2) that of Réaumur, Eisig, and most modern writers, that the shell is of the nature of an excretion, deposited like a cuticle on the outside of the skin, being formed simply of a number of calcareous particles held together by a kind of ‘animal glue.’ Leydig’s view is that the shell of the Monotocardia is a secretion of the epithelium, but that in the Pulmonata it originates within the skin itself, and afterwards becomes free.[335]