258 MISSISSIPPIAN BRACHIOPODA 



tween ill-defined lines, but is not depressed in a median sinus; the beak 

 prominent, pointed and incurved, pierced by a large, subovate foramen 

 which encroaches upon the umbonal region, only coming in contact with 

 the delthyrium at its apex; the delthyrium almost totally hidden by the 

 incurvature of the beak. Internally the dental lamellae are well developed 



FIG. 29. A series of ten cross-sections of the rostral portion of the shell of 

 Dielasma chouteauensis (X 2%), showing the dental lamellae of the pedicle 

 valve (A), and the development of the independent crural lamellae of the 

 brachial valve, with a transverse, muscle-bearing plate which becomes entirely 

 free from the floor of the valve anteriorly. 



and reach nearly one-fourth the length of the valve from the beak; the 

 muscular scars obscure, bounded laterally by a pair of shallow, narrow, 

 rounded furrows in the inner surface of the valve which diverge from 

 the beak between the dental lamellae and extend about two-thirds the 

 length of the valve from the beak. 



Brachial valve less convex than the pedicle, the greatest convexity 

 posterior to the middle, arched from beak to front with the greater 

 curvature posteriorly, the surface curving abruptly to the postero- 

 lateral margins and gently to the anterior and antero-lateral margins; 

 the median portion of the valve not differentiated as a fold or sinus and 

 scarcely even flattened ; the beak acutely pointed, incurved beneath that 

 of the opposite valve. Internally the socket plates are distinct from the 

 crural plates ; the crural plates originate as a pair of slight angular ridges 

 diverging from the apex upon the inner surface of the valve ; between 

 these ridges is a concave platform-like area for muscular attachment 

 which is at first wholly confluent with the inner surface of the valve, 

 anteriorly it becomes elevated a little above the floor of the valve laterally, 

 leaving a narrow, triangular, pocket-like cavity on each side, still further 

 towards the front this platform is elevated above the inner surface of 

 the valve through its entire width, leaving a broad, shallow cavity be- 

 tween its under surface and the inner surface of the valve, the anterior 

 extremity reaching one-third or more of the length of the valve from 

 the apex. 



Surface of both valves, at least in well preserved exfoliated examples, 

 marked towards the lateral and anterior margins by exceedingly faint, 



