182 MISSISSIPPIAN BRACHIOPODA 



shell, anteriorly it is broad and shallow, its surface is curved towards the 

 opposite valve, and its anterior margin is produced more or less conspic- 

 uously beyond the lateral margins ; the beak is pointed, only slightly in- 

 curved and is produced posteriorly beyond that of the opposite valve ; 

 the delthyrium is broadly triangular and is partially closed by deltidial 

 plates, with a subcircular foramen at the apex ; plications simple, suban- 

 gular, becoming faint or obsolete close to the beak, fourteen to twenty in 

 number, from three to six of which occupy the median sinus, the first two 

 or three on each side nearest the postero-lateral margins are very faint. 

 Internally the teeth are supported by short dental lamellae which do not 

 continue anteriorly beyond the articulation of the hinge. 



Brachial valve more strongly convex than the pedicle, the greatest 

 depth anterior to the middle and in adult shells near the anterior margin ; 

 from the point of greatest convexity the surface curves more or less ab- 

 ruptly to the anterior margin and more gently to the posterior extremity, 

 laterally the surface curves somewhat abruptly to the postero-lateral 

 margins, but the median portion of the valve is somewhat flattened trans- 

 versely ; mesial fold scarcely defined except upon the more or less abrupt 

 anterior slope of the valve and even there it is scarcely differentiated 

 from the remainder of the valve ; the beak strongly incurved beneath 

 that of the opposite valve ; plications similar in form and number to those 

 of the opposite valve. Internally no cardinal process is present, the 

 hinge-plate is longitudinally divided to the apex of the beak and in the 

 extreme posterior portion of the valve it is supported by .a median septum 

 which is divided above, each division of the hinge-plate being supported 

 by one limb ; the divided median septum with its crural cavity be- 

 tween the divisions of the hinge-plate is extremely short and disappears 

 posterior to the dental sockets, anteriorly the median septum is free and 

 is reduced rapidly in height to a slight ridge along the bottom of the 

 valve which sometimes continues for one-half the length of the valve; the 

 divided hinge-plate becomes nearly horizontal anteriorly where its anterior 

 extremity is produced into the crura. 



The surface of both valves, aside from the plications, is nearly or quite 

 smooth save for a few rather inconspicuous lines of growth near the an- 

 terior margin of adult shells. 



Remarks. This species is one of the abundant forms in the fauna of the 

 Salem limestone. It was originally described from near Alton, Illinois, 

 and in some of the Salem limestone localities in the bluffs above that city, 

 it is particularly abundant. It is a variable shell, the variations being 

 exhibited in the dimensions, convexity of the valves, fold and sinus, and 

 number of plications. It differs from C. grosvenori, which occurs com- 

 monly in the same fauna in Indiana, in its larger size, stronger and more 

 angular plications and less globular form. 



