256 



genera they belong to may be determined, or the characters 

 of new ones ascertained ; and this is the more desirable, as 

 they appear from their localities to be marine inhabitants, 

 rather than land shells." * 



In consequence of the agreement in form of the first 

 mentioned shell with the latter ones, they become all in- 

 volved in the same ambiguity ; possessing the exterior cha- 

 racters of the snail-shells of the present day, and being 

 imbedded among the earliest shells, those which possessed 

 the power of rising and sinking themselves in the water. 

 But a question here arises, and it is presumed that on these 

 subjects, involved in the darkness of distant ages, questions 

 and conjectures founded on analogy may be allowed: May 

 not these shells have been actually of a genus different from 

 helix ; and might not they have been furnished with such a 

 structure as gave them the same powers as were possessed 

 by their associates ? 



This conjecture derives support from the fact, that shells 

 of such a description exist even now in the seas of the v/arm 

 climates: such is ianthina, (PL v. fig. 23.) This shell had 

 always been considered as a helix by Linnaeus, and by his 

 successors, until the time of Lamarck, who determined it to 

 be necessary to place it in a distinct genus. He ascertained 

 that the inhabitant of this shell differed essentially from the 

 snail. That organ which would by its situation be con- 

 sidered as the foot in the snail, was found not to be formed 

 for crawling, but for swimming ; being covered with air 

 bladders which could be filled or emptied agreeable to the 

 impulse of the animal. This shell, which is very thin, with 

 four whirls in its spire, is, like the fossil shells of the same 

 form found in the mountain limestone, striated both longi- 

 tudinally and transversely, and, as if to diminish its gravity, 



* Min. Conch. Vol. ii. p. 159. 



