BRACHIOPODA. 817 
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 traversed 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 lamelle near their ante-lateral curvature. 
The mode of attachment 
is peculiar, the crural lam- 
elle bending upward and 
then abruptly downward, 
greatly widening at the 
line of contact and touch- 
ing the spiral ribbon only 
at its outer margin. The Fia. 399.— Diagram of Atrypa reticularis ; showing 
_ the form and structure of the jugum and the mode of 
demarkation between the attachment of the crura to the hinge-plate and the 
crura and the ribbon of Primary lamella. 
the coils is, therefore, very distinct. The spirals have, in a gen- 
eral 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 convergent, so that 
the approximate surfaces of the cones are flattened. The basal 
section of these cones is hemicordate, the anterior extremity 
being much the narrower, but the upper volutions are more 
nearly elliptical. The ribbon is broad, being conspicuously so 
on the anterior curves of the first few volutions, each one 
extending considerably beyond the next following. These 
anterior curves may be more or less distinctly fimbriated. The 
jugum is composed of two processes which are continuations of 
the primary lamelle without angulation. These processes are 
situated posteriorly, directed toward the center of the shell, and 
are, in effect, the starting points of the spirals. They have the 
following structure: The ribbon maintains its usual width for a 
considerable distance within the point of attachment to the 
crura, then narrows rather abruptly, the processes ascending as 
they approach each other. Their terminations in mature shells 
are broadened, thickened, erect and recurved at the tips, having 
a clavate appearance. In immature growth-stages or undeyvel- 
103 69 
