478 TERMINOLOGY OF 



outer lip alone. But maturity brings with it a change in the 

 organs of the animal. The lobes of its cloak become more 

 Fi 98 developed, and ultimately very large ; so that, 

 one issuing from each side of the aperture, 

 they can cover the shell, and meet in the 

 centre of the back. These lobes are secretory 

 organs, and pour out an abundance of lime in 

 a vitreous state ; and, by their motions spread- 

 ing it over the outer surface, the shell is 

 thickened, assumes a form totally different 

 from its primary one, and dependent on the 

 new development of the soft parts, 



To the above forms most univalves may be 

 reduced. Intermediate shapes are expressed 

 by prefixing the diminutive sub to the adjec- 

 tive word, as subglobose ; or by combining two adjectives 

 together, as o vat o -fusiform. 



The whorl which contains the aperture is called the body- 

 whorl. It is the last formed, and is only finished with the 

 full maturity of the Mollusk. The number of whorls varies 

 of course with the age of the individual, but it seems to 

 be very uniformly the same in the individuals of the 

 same species. Adanson, however, asserts, that in Purpura, 

 Buccinum, and some other genera, the shell of the male has 

 usually more whorls than that of the female, and is at the 

 same time more gracile and elongated. The latter observa- 

 tion you may verify by examining our common whelks, in 

 which it is easy to distinguish the female by the bulged 

 contour of the body-whorl. 



On holding a shell with the aperture towards you and the 

 apex aloft, you will perceive that the whorls revolve from 

 right to left. These shells are said to be dextral (Fig. 96). 

 When the revolution of the whorls is in the contrary direc- 

 tion, the shell is sinistral.* Some dextral shells are occa- 

 sionally sinistral, and such a specimen is prized from its 

 rarity;-)- but I do not remember to have seen any sinistral 

 species with a dextral individual. 



* " You tell me, that it is generally concluded by philosophers, that the 

 reason of the usuall turn of snailes from the left to the right, is the like motion 

 of the sun, and that especially more nord-ward, there having not been hitherto 

 discovered any in our parts of the contrary turn to the sun's motion. But 

 this is not the only case, where they are out, who consult not the stores of 

 nature, but their own phancy." LISTER to RAY. Phil. Trans. 1669, p. 1014. 



f " My well-informed friend, Mr. Pratt, who obligingly arranged the 

 shells in the Ashmolean Museum, tells me, that he knew a French natural- 

 ist who had contrived to obtain a breed of reversed snails, which he sold 



