40 
rocks. In Case f, Sect. 10, and Case H, Sects. 2 and 10, and Case 
r, Sect. 3, will be found other forms and other genera. In the 
Chemung and Waverly only a few obscure forms are represented ; 
but in the Keokuk and Chester divisions of the Lower Carbon- 
iferous rocks, many forms of peculiar interest are exhibited, 
especially Archimedes; see Case K, Sects. 1 3, 6, 7 and 10. There 
are also a few shown under the New Jersey Cretaceous. 
BRACHIOPODA. 
The Braclliopoda are mostly small, shell-bearing, marine 
animals, which derive their class name, which means arm-footed, 
from the form of their breathing organs, which are often coiled in 
form of a spiral, and are in some cases long and slightly flexible, 
so as to be protruded beyond the shell, like arms or feet. They 
are the most abundant fossils in the Palaeozoic rocks in America 
and in the European Mesozoics, and may be found represented 
under all the groups. They are the lamp shells of the ancients, and 
may usually be distinguished from the other bivalve shells by their 
form, being bilaterally symmetrical from a vertical line extending 
from the apex to the base, while one valve is usually much larger 
and longer than the other. The following illustrations will convey 
an idea of their general form. 
