392 FIROLIDJE; ATLANTID.E. FOSSIL SHELL3. 



form, the body is much elongated, the foot presents the pecu- 

 liar structure above described, and the gills are either naked 

 or covered by a small conical shell. The latter is seen in the 

 Carinaria (Fig. 682) ; the shell of which presents a con- 

 siderable resemblance to that of the Argonaut ( 962) ; in fact, 

 by some naturalists it has been supposed that the shell inhabited 

 by the Cephalopod just mentioned is constructed by a Mollusk 

 allied to Carinaria, and that the Argonaut only makes use of 

 the empty shell as a protection for its body. In Firola there is 

 no sheh 1 . The animals of this family are inhabitants of the 

 warmer temperate and tropical seas ; several species are found 

 in the Mediterranean. 



1009. The ATLANTIC^ are provided with a large shell, with- 

 in which they can retract themselves at pleasure ; and the foot 

 sometimes bears an operculum, with which the aperture of the 

 shell may be closed. The shell greatly resembles that of the 

 Nautilus in form ; and when first discovered, the Atlanta was sup- 

 posed to be the living representation of the extinct Ammonites. 

 The Atlantidae also differ from the preceding family, in having 

 the branchiae inclosed in a regular dorsal chamber. They are 

 found, like the Firolidse, in the seas of warm climates. 



1010. To enter into any detailed account of the Fossil Re- 

 mains of this class, would be unsuitable to our present purpose ; 

 and it will be sufficient here to state some general facts in regard 

 to them. Remains of Univalve shells, of such a form and struc- 

 ture as evidently to have belonged to Gasteropod Mollusks, are 

 found in nearly every bed formed by the action of water, from 

 the very earfcest of those containing fossils of any kind, down to 

 the present time. In many instances, the fossil shells, even of 

 the most ancient beds, may be referred to genera which still 

 exist ; thus a species -of Buccinum is found in the rocks of the 

 Silurian system ; although they do not correspond with any 

 species now living, except in cases where there is reason to believe 

 that the deposit was formed at a comparatively recent period. 

 The Conchologist is generally able to determine, by an examina- 

 tion of the shell, whether it was formed by a marine or & fresh- 

 water Mollusk ; and in this manner he often receives important 



