i ; Case f, Sect. 8 ; Case G, Sects, i and 7 ; Case h, Sects. 4 and 
11. In the Hamilton group they are very numerous ; see Case 1, 
Sects. 10 to 13, and Case j, Sects. 1 to 3. Alcove Case, No. 8, is 
partially devoted to figured specimens of this group. The Che- 
mung group also affords large numbers of them ; see Case L, 
Sects. 12 and 13 ; Case m, Sect. 1, and Alcove Case, No. 10. 
Among the Catskill rocks there is a single species represented, 
Ammgema Catskillensis, Vanuxem, Case m, Sect. 3, which is prob- 
ably of fresh-water origin, and if so, is the earliest representative 
of fresh-water Mollusca on the American continent. The Car- 
boniferous epochs afford only a few species each, but in the Cre- 
taceous and Tertiary they are extremely abundant, and in the 
latter are exceedingly like those of the present seas ; see Case P, 
Sects. 8, 10 and 11. 
GASTEROPODA. 
This group comprises the modern Snails, or coiled and spiral 
shells, called univalve, as they consist of only one piece, although 
there is often a covering for the aperture. They are either Marine, 
Fresh-water or Land. They make their appearance in the Pots- 
dam epoch, in the genera Platyceras; Holopea and Ophileta, and 
constantly increase in numbers to the present time ; although in 
some of the epochs they were but sparingly represented over cer- 
tain regions. The older forms were mostly of the sections 
Holostomata and Schizostomata, and were to a great extent 
vegetable feeders. The Siphonostomata, which are generally 
carnivorous, are not abundant until the Mesozoic time, although 
the genus Fusispira, in the Trenton epoch, presents the general 
features of shells of this group. The Pui.monifera (air breathers 
or land snails), appear first, so far as at present known, in the 
Devonian epoch, and are most abundantly represented at the 
present time. Examples of two minute forms from the Coal epoch, 
Pupa Vcnnilionensis and Daiysonella Meeki, may be seen in the 
Dana's Manual series of that epoch. 
The section Heteropoda, including Bellerophon, Bucania, 
Cyrtolites, &c, is represented under the Trenton, Upper Helder- 
berg, Hamilton, Chemung- and Carboniferous. The following 
figures give a few forms of the Gasteropoda. 
