320 NATURAL. HISTORY. 
class has some species that live in fresh water, and some 
even that live on land. I will notice each of these groups. 
551. Of the Cephalopoda, the only few existing species 
that have a shelly covering are the Argonauts and the 
Pearly Nautilus. There are, however, many fossil shells 
found which must have belonged 
to animals of this group. The 
Ammonites, commonly called 
Snake Stones, of which a speci- 
men is given in Fig. 251, are the 
most abundant of these, there hay- 
ing been described over five hund- 
red species. These are found in 
various kinds of rocks, and are of 
various sizes, some reaching a di- 
ameter of even four feet. 
552. The arms of some of the Cephalopods are very 
long. This is the case with the Cuttle-fish, one of the 
most singular of animals, seen in Fig. 252. Its body is 
Fig. 251.—Ammonite. 
Fig. 252.—Cuttle-fish. 
soft, and is covered only with a leathery skin. From 
around its mouth extend eight long arms, which have on 
them great numbers of little suckers, by which it can 
