250 MISSISSIPPIAN BRACHIOPODA 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, arched from beak to front along the median line with the curva- 

 ture becoming regularly more convex posteriorly, the surface curving ab- 

 ruptly from the median line towards the postero-lateral margins and be- 

 coming inflected towards the cardinal extremities, curving more gently to 

 the antero-lateral and anterior margins ; mesial sinus obsolete and the 

 mesial portion of the valve not differentiated from the lateral slopes ; the 

 beak prominent, incurved, pierced by a subcircular foramen which en- 

 croaches wholly upon the umbonal portion of the valve, only coming in 

 contact with the delthyrium at its apex ; delthyrium largely occupied by 

 the incurved beak of the opposite valve, the remaining space closed by 

 the deltidial plates. Internally the dental plates are well developed and 

 extend anteriorly from the beak for nearly one-fourth the length of the 

 valve; the muscular scars obscure or obsolete in smaller specimens, but 



FIG. 25. A series of six cross-sections of the rostral portion of the brachial valve 

 of Crancena globosa (X 2%), showing the concave hinge-plate which is per- 

 forated posteriorly, and the crural ridges upon the hinge-plate. 



on the internal casts of larger and older examples they are suggested by 

 more or less indefinite markings radiating from the beak. 



Brachial valve a little less convex than the pedicle, the greatest convex- 

 ity near the middle, the surface regularly arched from beak to front 

 along the median line with the curvature a little more convex posteriorly, 

 from the mesial line the surface is rather abruptly curved to the postero- 

 lateral margins and somewhat more gently to the antero-lateral and an- 

 terior margins; the mesial portion of the valve not differentiated from 

 the lateral slopes ; beak acutely pointed, incurved beneath the base of the 

 pseudodeltidium of the opposite valve. Internally the socket plates are 

 joined transversely by an elongate, concave hinge-plate which is perfor- 

 ated at its apex, anteriorly beyond the articulation of the valves the 

 lateral attachments of the hinge-plate to the inner surface of the shell 

 retreat from the lateral margins of the valve and these continuations of the 

 socket plates are reduced to mere ridges between which, upon the eon- 

 cave side of the hinge-plate, a pair of crural ridges originate ; the crural 

 ridges diverge anteriorly and are produced into the bases of the crura, 

 they are still connected transversely by the anterior extremity of the 

 hinge-plate after connections with the inner, lateral surfaces of the valve 

 have disappeared; the brachidium is short and Dielasma-like with rather 

 widely divergent descending lamellae and sharply recurved ascending 

 lamella. 



