BRACHIOPODA. 
165 
valve, and are hence unsupported by lamellae. The muscular impressions are 
sharply defined; the triangular pedicle-scar is followed in front, by a median 
elongate double scar of the adductors, outside of which are strong, radiately 
striate, flabellate diductors, which frequently extend beyond the middle of the 
valve. 
Brachial valve convex or rotund in the middle, with a median fold which is 
rarely developed except toward the anterior margin. Beak incurved and con¬ 
cealed. No cardinal area. The hinge-plate is composed of two diverging pro¬ 
cesses which may or may not meet at the apex. Each of these processes is 
obliquely grooved, forming an inner and outer lobe. The latter forms the 
upper portion of the socket wall which is curved downward and unites with 
the lateral surface of the valve, forming a broad dental socket which is trav¬ 
ersed by an oblique crenulated ridge. The inner lobes of the hinge-plate are 
short, their extremities free, bearing the crura.* 
These crura are long and narrow, diverge laterally and are attached to the 
primary lamellae near their ante-lateral curvature. The mode of attachment 
Diagram of Atrypa reticularis ; showing the form and structure of the loop and the mode of attachment of the 
crura to the hinge-plate and the primary lamell®. (C.) 
is peculiar, the crural lamellae bending upward and then abruptly downward, 
greatly widening at the line of contact and touching the spiral ribbon only at 
its outer margin. The demarkation between the crura and the ribbon of the 
coils is therefore very distinct.* The spirals have, in a general sense, their 
bases parallel to the inner surface of the pedicle-valve and the apices directed 
toward the deepest point of the opposite valve. Their axes are more or less 
* In the mode of attachment of the crura, as heretofore represented, they have been made to appear as 
if derived from the outer lobes of the hing-e-jdate. See Palaeontology of New York, vol. iv, pi. liiiA, 
figs. 22, 25. 
