Sects. 4 and 6 ; Hamilton, Case j, Sects. 3 and 4. The Coal Measures 
afford a few only, but in the Cretaceous and Tertiary there are 
many, especially in the Eocene, from Claiborne, Ala., where they 
are nearly as perfect as recent shells ; Case P, Sect. 1 2. The 
European collection affords a few scattered examples, but in the 
Paris basin, Eocene, Alcove Case, No. 16, are many fine ones. 
Fro. 33. — PTEROPODS. 
PTEROPODA. 
This group is composed entirely of open sea animals, the most 
of which are destitute of shells, and consequently are not found 
fossil. The shells, when present, are 
small, usually thin and fragile, and as 
fossils generally imperfect. They are 
represented as fossils by only a few 
genera; Hyolithes, Tentaculites and 
Conularia being the principal ones. 
Tentaculites are so abundant in the 
Lower Helderberg period, as to form 
layers of rock many feet in thickness, 
in Albany and Schoharie counties, in 
New York. The following figures 
f,c. 33: tt ,Conmaria Trent™. re P resent specimens of two of these 
ensis; /*, Tentaculites gyracan- genera, 
thus, natural size and enlarged. 
Species of Hyolithes may be found under the Potsdam forma- 
tion, Case B, Sects. 1 and 2. Conularia is represented in Case c, 
Sect. 4, and Case e, Sect. 7 ; Case o, Sects. 2 and 7 ; Case J, Sect. 
4, and Case n, Sect. 4. Tentaculites may be found in Case G, 
Sect. 2, and Case j, Sect. 4. 
CEPHALOPODA. 
This, the highest group of Mollusca, is divided into two principal 
branches: Tetrabranchiates (four lunged) and Dibranchiates 
(two lunged). The first is represented in the present seas by the 
genus Nautilus, and the last by the Cuttle-fishes, Squids and by 
the Argonaut. As fossils the first division is known from near 
