257 



possesses no solid columella. * When the sea is calm, the 

 ianthines, it is said are seen, in great numbers, swimming 

 on the surface of the sea with their shells reversed, by means 

 of their distended air-bladders ; but on the sea becoming 

 agitated, the air in the bladders is absorbed, the animals 

 contract within their shells, and the whole immediately 

 sink. 



This power is, indeed, possessed by other shell animals 

 of the present day, besides the ianthines ; the tymncea 

 stagnalis (Le Grand buccin de Geoffroy) is known to swim 

 on the surface of the water in a reversed position, and, 

 when desirous of returning to the bottom, it compresses 

 itself within the shell, thus expelling the air which had 

 supported it ; and, by this simple process, sinks immediately 

 to the bottom. 



The opinion that these shells of early creation, bearing 

 the external characters of snails, might have been natant 

 animals, having the power of swimming to the surface, like 

 their associates, derives some confirmation from analogy ; 

 since the multilocular, natant shells are not only found in 

 their own peculiar forms, as nautili, ammonitce, &c. but have 

 also assumed the peculiar form, as in turrilites, of unilocular, 

 turriculated shells, which do not appear to have been 

 created until a much later period. 



In the lias formation, which succeeds to the mountain 

 limestone, the contained fossils differ considerably from 

 those which preceded them : only a few species of some of 

 the multilocular univalves, and of the terebratulas, being 

 here discoverable. The fossil shells found in this formation 

 chiefly consist of bivalves of the genus ostrea, gryphcea, 

 plagiostoma, plicatula, avicula, mi/a, and cardita, with the 

 single trochiform shell, trochus anglicanus of Lister, not 



* This is also the case with cirrus, and, perhaps, with euomphalus. 



Ll. 



