452 Dr. J. E. Gray on "a New 



A protecting organ of a gland, or a gland itself, becoming 

 shelly would be an entirely new fact in malacology ; and the 

 notion should not be entertained without very strong reasons,* 

 of which M. Favi-e gives none. 



All true shells are secreted by the mantle of the mollusk, 

 and not by any other part of the animal. The operculum of 

 Univalves, which is the analogue of the second valve in the 

 Bivalves, has a peculiar mantle on the foot of the animal for 

 its secretion ; and when the operculum is formed of several 

 layers (that is to say, when its inner and outer surfaces are 

 covered with an additional calcareous coat) the outer coat is 

 secreted by a peculiar lobe of the mantle, as the outer coat of 

 the cowrie, Marginella, &c. is secreted ; and I have no 

 doubt that the outer coat of AjJtychus is secreted by a lobe of 

 the pedal mantle, like the outer coat of the operculum of 

 Gasteropods. 



The only instance that has occurred to me of a body se- 

 creted by a mollusk having the slightest resemblance to a shell, 

 and yet not being secreted by the mantle of the animal, is 

 that of the three shelly plates that encase the gizzard oi Bulla 

 Ugnaria and B. aperta. These plates are only the hardening of 

 the cartilaginous tubercles that are found in tlie stomach of 

 A'phjsia and other allied genera, and have not the structure 

 or texture of true shells ; tliey certainly bear no resemblance to 

 the shells of Aptychus^ \\ hich, as M. Favre describes them, 

 have the regular texture of shells. 



The structure of the Aptychi that I have examined, as well 

 -as the account of it given by M. Favre (p. 365), is quite the 

 same as that observed in many opercula of Univalve shells. 



It certainly is against all my experience of fossil shells 

 (which has been extensive) if the Aptychus is a fossil nida- 

 mentary gland, or that a soft glandular part should be fos- 

 silized so as to produce a body formed of three layers, each 

 with a peculiar structure, and that the structure which they 

 produce by becoming fossilized should be similar to the 

 structure observed in opercula^ which are often formed of three 

 layers, as M. Favre describes them. The reasons which he 

 gives that they cannot be opercula show M. Favre's slight 

 acquaintance with the structure and economy of living 

 Mollusca ; for otherwise he would have known that the majority 

 of opercula, although found in the aperture and protecting the 

 animal, evidently " could not have served to close the aperture 

 of the shell." 



M. Favre observes : — " The shell of Nautilus is composed 

 of two layers — an external layer formed of an aggregate of 

 cells of different sizes, and the largest of which are those 



