58 



the Cowrey the testaceous fluid is deposited from the outside, in layers at 

 different intervals, whilst in the Olive it is secreted in layers simultaneously 

 at the lip, and the porcellanous surface of the shell is preserved in its course 

 of retrovolution, as well as after maturity, by a reflexion of the ventral disc, 

 in a manner somewhat analogous to the reflected mantle of the Cowrey ; 

 bearing no part in the formation of the shell, however it may minister to 

 its preservation by the tendency of its viscid humour. 



It now remains to ascertain whether the distinction here assigned to the 

 calcifying functions of these mollusks, is borne out by the physical condition 

 of their shells at different stages of growth. Each period in the life of 

 the Cowrey (as described at page 40) is illustrated by a different design of 

 colouring ; the shell ceases its course of retrovolution long before arriving 

 at the adult state, and increases in substance and variety of colouring by 

 the outward expansion of the mantle. It is not so in the case of the Olive ; 

 the shell of this mollusk exhibits the same appearance at all stages of 

 growth, the different layers of colouring matter must of necessity be depo- 

 sited simultaneously, because the organ of calcification reaches only to the 

 lip, from which the shell gradually recedes in its progress of growth. The 

 Olive-shell has no periodical change of colour to mark its advancement, 

 yet it will be found on removing the outer coat by chemical agency, that 

 there is a superincumbent layer of different colour and design. The exter- 

 nal coating of the Oliva virlculus is of an obscure milky blue colour, but 

 the removal of this layer by the application of an acid, reveals a dark ashy 

 ground, sprinkled with numerous triangular opal-like dashes ; the removal 

 of the outer layer of the Oliva Brasiliensis exhibits a longitudinally striped 

 pattern, and other varieties of design may be found in different species. 



The Olives are confined exclusively to the tropical regions, we have none 

 on our own coast, nor is there a single species in any part of the Medi- 

 terranean. They are described by M. Quoy, in the ' Voyage de 1' Astrolabe ', 

 as possessing an unusual degree of activity, and able to regain their position 

 when placed upon their backs ; and Mr. Gray observes in Beechey's Voyage, 

 that they live partially buried in the sand, though easily taken by lines 

 baited with flesh. 



Species *. 



1. acuminata, Chemn. 5. aniomina, Duclos. 9. australis, Duclos. 



2. alectona, Duclos. G. araneosa, Lam. 10. avellana, Lam. 



3. anaxora, id. 7. atelina, Duclos. 11. bicincta, id. 



4. angulata, Chemn. 8. auricularia, Lam. 12. biplicata, Sow. 



* The species of Oliva have been so inefficiently described, and so many nonsensical names 

 have been introduced by M. Duclos, that a complete monograph of the genus with a change of 

 nomenclature, would, I think, be an admissible and most acceptable contribution 



