BracwHioropA. 839 
which converge, and uniting, are supported by a short median 
septum, forming thus a well-defined spondylium. 4 
In the brachial valve the hinge-structure is similar to that of 
CamarorecutA, the crural plates converging and forming a short, 
very small median cavity, which is supported by a long septum. 
The crura are short and the lateral divisions of the hinge-plate 
small. No cardinal process exists. 
Type, Camarella Volborthi, Billings. 
Distribution. Cambrian (?)— Lower Silurian. 
Parastrophia, Hall. 1893. 
(Plate 45, figs. 20-27.) 
Shells broad, tranversely oval in outline; surface with low 
rounded plications which are stronger on fold and sinus, 
apparent on the lateral slopes’ only near the margins of the 
valves. Brachial valve much the more convex, its umbo pro- 
jecting conspicuously beyond that of the pedicle-valve. Cardinal 
margin straight and moderately long; no evidence of cardinal 
area in either valve. 
In the pedicle-valve the delthyrium is broadly triangular and 
is usually filled, partially or wholly, by the beak of, the opposite 
valve. On the interior the dental lamella make a strong 
‘spondylium which reaches almost to the bottom of the valve, 
being supported by a very low median septum extending nearly 
one half the length of the shell. 
In the brachial valve there are two vertical crural plates not 
connected by a cardinal process. These are slightly convex on 
their inner surfaces and at their point of greatest convexity they 
unite with two longitudinal and gradually convergent lamelle, 
which form a spondylium narrower than that of the opposite 
valve and supported by a very low median septum somewhat 
longer than that of the pedicle-valve. 
Type, Parastrophia hemiplicata, Hall(sp.). (Trenton limestone. 
Distribution. Lower — Upper Silurian. 
Anastrophia, Hall. 1867. 
(Plate 45, figs. 28-35.) 
Shells with strongly reversed convexity. External surface 
covered with numerous fine and sharp dichotomizing ribs, extend- 
ing to the apices of beaks and frequently crossed by delicate con- 
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