154 MISSISSIPPIAN BRACHIOPODA 



anteriorly or sometimes with a slight, ill-defined median sinus; the beak 

 small, a little incurved, extending posteriorly only slightly beyond that 

 of the opposite valve; cardinal area small, concave, with the curvature 

 increasing towards the beak, the inferior, flatter portion sloping poster- 

 iorly at an angle of from 50 to 60 degrees to the plane of the valve, the 

 lateral margins sharply defined ; the delthyrium broadly triangular, much 

 wider than high, its width at the base usually equal to about one-third 

 the total length of the hinge-line. Internally the cardinal teeth are of 

 moderate size and are widely divergent; the muscular scars are large, 

 with their margin extending beyond the middle line of the valve, flabellate 

 in outline, the adductor scars occupy a central, subcircular area, nearly 

 one-third of the total length of the entire scar, which is distinctly raised 

 above the general surface of the valve anteriorly and in front of which a 

 gradually decreasing median ridge continues nearly to the limits of the 

 diductor scars ; the unpaired, median pedicle scar is very large, occupying 

 the entire rostral portion of the shell and encroaching upon the adjacent 

 scars in front from which it is sharply differentiated; the inner margin 

 of the valve distinctly crenulate laterally and anteriorly. 



Brachial valve equally or a little more convex than the pedicle, with 

 the greatest convexity posterior to the middle, the surface curving more 

 abruptly to the cardinal margin, more gently to the lateral margins and 

 most gently to the anterior margins, usually a little compressed towards 

 the cardinal extremities; a shallow, rounded, rather narrow, ill-defined 

 median sinus originates in the umbonal region and continues to the an- 

 terior margin; the beak is inconspicuous, very obtusely pointed and not 

 incurved ; the cardinal area much smaller than that of the opposite valve, 

 lying in nearly the plane of the valve. Internally the cardinal process is 

 erect, rather prominent, and obtusely pointed at the summit, the posterior 

 face is subcarinate along its median line, becoming obscurely trilobate 

 towards the summit, and is nearly vertical to the plane of the valve, the 

 anterior face is transversely rounded and slopes rather abruptly to the 

 front and is extended anteriorly along the inner surface of the valve as a 

 rounded median ridge which flattens out and becomes obsolete at the 

 anterior extremity of the muscular scars ; from either side of the cardinal 

 process the socket plates originate and diverge widely, from the outer 

 surface of each one a conspicuous lateral process limits the dental sockets, 

 but beyond these processes the plates are produced into free, crura-like 

 extensions ; the muscular scars are distinctly quadripartite and are sub- 

 quadrangular in outline, their anterior margin reaching to about the 

 middle of the valve ; the inner margin of the valve is distinctly crenulate 

 anteriorly and laterally. 



The surface of each valve is marked by fine, rounded, radiating costae, 

 three or four of which occupy the space of one millimeter. These costee 



