BRACIIIOrODA. 
33 
which is known hy the term cardinal process. To this process 
are attached the pair of great cardinal muscles, which open 
the shell, by pulling on this process—the fulcrum being the 
hinge and its teeth. 
The characters in Strophomena have been illustrated by 
Prof. King and myself, and the genus accepted by our chief 
authority on Brachiopod shells, Mr. Davidson. In this genus, 
though a few have somewhat of the bent shell and transverse 
form of Leptcena, the great majority are flat semi-oval shells, 
as long as broad, and with a hinge line invariably equal to 
the greatest width of the shell, often a little more. Internally 
the muscles are of a quadrate form, and much expanded later¬ 
ally as well as lengthwise ; and the cardinal process is very 
distinct, large and double. 
Or this proper is seldom flat, and generally has one valve much 
more convex than the other. The hinge line is often not as 
wide as the shell. The form is compact, not extended. The 
muscular scars are quadrate, not expanded either lengthwise 
or sideways in either valve. And there is a single distinct 
linear cardinal process. 
These are tangible characters, and combining, as they do, 
external and internal peculiarities, offer safe as well as conve¬ 
nient distinctions for a large group of shells abundant in 
species, and very much alike in general appearance. TV ithout 
this clue it is a hopeless labyrinth of variable forms, do all 
this it maybe added that Leptcenci, as above defined, is restiict- 
ed to Silurian rocks : *S 'trophomena is Silurian and Devonian; 
while Orthis ranges through all the paleozoic formations. 
Leptana, has but few, perhaps twenty or thirty species known. 
Both the other genera reckon their specific forms by hundreds. 
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