INTRODUCTION. 3 
fresh water shells; and these fossils are often found so 
entire, especially in the tertiary formations, that when dis- 
engaged from the matrix, they have all the appearance of 
having been just procured from the sea.” 
The great utility of Fossil Shells in a geologieal point of 
view, is the perfect condition in which they are generally 
found, so much so, that the species can readily be distin- 
guished, which is seldom the case with other fossils. 
Hence their service in identifying strata. Some of the 
genera are peculiar to the land, others to the fresh water, 
and others, again, to the sea, by means of which we are 
led to a knowledge of marine and fresh water deposits. 
EXPLANATION OF THE DIFFERENT PARTS 
OF SHELLS, REPRESENTED ON PLATE I. 
Criass First. — Mottusca or UNIVALVES, OR SHELLS 
WHICH ARE COMPLETE IN ONE PIECE. 
The shells of this class are termed by Lamarck Mollusca. 
They are, for the most part, spiral ; but a few genera are 
destitute of a regular spire. The leading characteristics of 
this class are the general form of the shell, the particular 
shape of the aperture, the length and construction of the 
pire and beak, and the structure of the outer and pillar 
ips. 
Apex. Thie tip of the spire, or the extremity opposite 
the base. Fig. 1. a. and fig. 17. a. In shells which are 
destitute of a regular spire, such as the ParELLm, &c. the 
vertex or highest portion is thus designated. 
Base. In shells with a beak, or rostrum, the base is the 
extreme end of this organ. Fig. 7. 6. and fig. 17. 5. In 
those which are destitute of a beak, it is the lower portion 
of the body volution. Fig. 21. a. 
Body is the lower volution, or that portion of the shell 
in which the aperture is situated. Fig. I. c. and fig. 14. ¢. 
Spire consists of all the convolutions of the shell but the 
lower one. Fig. 7. d. 
Suiure of the spire is that hollow spiral groove, or raised 
