42 
The Brachiopoda are not only more numerous as fossils than any 
other group of animal remains, but are the best guides in 
determining the position and age of the strata, from the earliest 
appearance of life to the close of the Carboniferous formations. 
From that period onward they become rapidly less numerous, 
especially in Genera, to the present time, when there are only 
about one hundred and twenty living species known. 
MOLLUSCA. 
The true Mollusca are the real shell-bearing animals, whether 
Marine, Fresh-water, or Land. This section of the sub-kingdom is 
divided into four principal classes : La?nellibranchiata, Gasteropoda, 
Pteropoda and Cephalopoda ; each of which is seperately considered 
below. 
LAMELLIBRANCHIATA. 
This class comprises the bivalve shells, such as Oysters, Clams, 
Cockles, Scallops and Fresh-water Mussels. They differ from the 
Brachiopods, which are also bivalve, in being symmetrical on the 
opposite sides of the line of junction of the valves, and in the 
valves being inequilateral of themselves ; while with the Brachio- 
pods the reverse is the case. Shells of this group first make their 
appearance in the Cambrian (Potsdam), in America, where they 
are represented by two species only, one a nuculoid shell 
(Fordella), the other an undescribed genus. They gradually in- 
crease in number from that period to the present time, when they 
appear to have attained their maximum development. They are 
comparatively abundant in the Trenton, Hamilton and Chemung 
epochs, but not numerous in the other parts of the Palaeozoic 
formations. The following illustrations will give an idea of their 
forms. 
