FORMATION OF SHELLS. 429 



they deposit a considerable number of layers of shelly mat- 

 ter, either on the lip or on the pillar : these layers are 

 formed of laminae disposed in the same manner as those of 

 the inner layer of the body of the shell, and, indeed, may 

 be considered as repetitions of this part. 



The animals of some genera, as for instance Cyprasa, 

 Ovula, Erato, and Marginella, deposit, at the same period, 

 on the outside of their shells an additional coat covering the 

 lip and back, which may also be considered as a continuation 

 of the inner layer, since it agrees with this layer in the posi- 

 tion of the laminae : it changes its character, however, on 

 becoming external, being harder, more compact, and often 

 differently coloured. In all these cases the sides of the 

 mantle of the adult animal are expanded into wing-like pro- 

 cesses, which are reflected over the shell, and deposit this 

 additional layer. In the Cypraea, Erato, and Ovula, in 

 which the lobes of the mantle are large and nearly meet, 

 there is left a differently coloured line at the place of their 

 junction, which, on account of the left lobe of the mantle 

 being usually the largest, is generally situated on the right 

 side of the back, and is called the dorsal line : this line is 

 not observed on the Marginellas, the lobes of the mantle 

 being in them less developed, and leaving a broad uncovered 

 space on the back. 



It is probable that the polished coat of the Struthiolaria 

 oblita is formed in the same manner, but the animal of this 

 shell has not been examined. From the reflected form of 

 the lip, however, in other species of that genus, and from 

 the edges of the mantle in the other genera of the family of 

 Strombidaa (to which Struthiolaria evidently belongs), be- 

 coming developed when the animal attains its full growth, as 

 is proved by the expansion of the lips of the Strombi and 

 the lobed form of those of the Pterocerae and Rostellariae, 

 it is easy to suppose that its lobes may even become com- 

 pletely reflected. 



The species of Cymbium, when fully grown, or under 

 particular circumstances, are often covered externally with 

 an additional glazed coat, which is apparently deposited by 

 the foot ; as the animals of these shells are not provided 

 with large lobes to their mantle. This coat being deposited, 

 over the periostracum, is liable to be broken off. 



There is deposited on the sides, and more especially in 

 the posterior part, of the cavities of the upper whorls of 

 many of the spiral univalves, a transparent calcareous con- 

 cretion, which lines, and more or less fills up the cavity. 

 This deposit may be observed lining the cavity of the upper 



