34 GEOLOGICAL SURVEY 
is either in the centre or towards the inner edge. In the Ammonitide the septa are waved and 
foliated on the margin, and the siphunculus is placed towards the outer edge or back of the shell. 
In the first of these groups we find three common genera. 
Nautilus, which has contiguous spiral whorls, convoluted in the same plane, and the siphunculus 
central. 
Orthoceras, a straight, elongated chambered shell, with septa concave towards the opening, and 
siphunculus central. , 
Cyrtoceras, which only differs from Orthoceras in being slightly curved. 
Here also must be placed the beautiful little shells called Polythalamia, (composed of numerous 
cells,) in which the divisions between the shells are punctured and appear as if covered with dots ; 
these, although often microscopic, are sometimes sufficiently large to be visible to the naked eye. 
They are known to form a large portion of the chalk beds of England, and the marl of the Ashley 
abounds in the most beautiful forms, which are also of great size. In the cabinet of F. 8. Holmes, 
[usq. in Charleston, a fine suite of well preserved specimens may be seen, which were taken from 
the Artesian well in that city. 
The Ammonitide include several interesting genera. 
Ammonites—shell discoidal, whorls contiguous, septa lobed, margins foliated, lip thick, siphuncle 
at outer or dorsal edge. 
Goniatites differs from the last genus principally in having the septa without notches or 
foliation. 
Crioceras differs from Ammonites in having the whorls not contiguous. 
Ancyloceras has the whorls like Crioceras, but the last one is prolonged and again recurved. 
Toxoceras has a slightly curved shell. 
Hamites—whorls not contiguous, eliiptic, last whorl recurved. 
Ptychoceras—shell bent like a siphon. 
Baculites—shell straight, tapering, upper part without septa. 
Scaphites—whorls rolled up and partly concealed by the last whorl, which is also rolled up like 
a hood, and has no septa. 
Turrulites — coiled spirally around an axis, conical, umbilicated, septa sinuous, siphunculus 
continuous, near the outer edge or next to the suture. The fossils of this class are among the 
oldest of animated beings. 
Of Nautilidee only two genera have come down to us; of the other groups, not one, although 
they existed in vast numbers during all the geologial periods, up to the termination of the Creta- 
ceous system, and a few have extended into the Tertiary rocks. 
Class Gasteropoda.—The G'asteropods derive their name from the muscular foot beneath the 
body, by means of which they move about. They are marine, fresh water and land animals. 
When provided with calcareous coverings they are composed of one piece—hence called univalves. 
The land and fresh water species are mostly herbivorous, but the marine species are carnivorous. 
The depredations committed on oyster-banks by Buccinum cinereus is well known. This animal, 
like many others, is furnished with a boring apparatus. Circular perforations may be seen in fossils, 
the work of these predatory animals. The shells of this class, in fresh water and land species, 
have the aperture entire, and without canal or notch. On the contrary, the marine species 
