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 



FlO. 153. 

 X>\Kgnm ot Atrypa reHeularit i showing the rorm and stmctare of the loop and the mode of attachment of the 

 enira to the hinge-plate anil the primary iameiiie. (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 deniarkation between the crura and the ribbon of the 

 coils is therefore very distinct. The spirals have, in a general sense, their 

 ba.ses p.arallel 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 i-epi-esented, they have been made to appeal' as 

 if derived from the outer lobes of the hinge-plate. See Palaeontology of New York, vol. iv, pi, liiiA, 

 figs. 22, 29. 



