198 MISSISSIPPIAN BRACHIOPODA 



or a little back of the middle of the valve and becomes rapidly deeper 

 anteriorly and is moderately produced in a broad, rounded extension in 

 front, where its width is about one-half the total width of the shell ; it is 

 usually nearly flat across the bottom in front with the sides sloping out- 

 ward or rarely nearly vertical ; the beak is acuminate and sharply pointed 

 and is strongly incurved, coming nearly in contact with the umbo of the 

 opposite valve ; the delthyrium is nearly filled by the beak of the opposite 

 valve, the deltidial plates and foramen are obscure and have not been 

 clearly recognized in any of the specimens studied; the plications are 

 simple, angular, rather coarse, and originate at the beak, three or rarely 

 four occupy the bottom of the sinus with sometimes an additional one 

 upon each sloping side, with from four to six upon each lateral slope 

 of the valve, the plications of the lateral slopes nearest the margins of the 

 sinus are the strongest and most angular ones on the valve. Internally 

 the hinge-teeth are weak and are supported by a pair of short, thin, 

 dental lamella;. 



900 



Q Q 



FIG. 11. A series of six cross-sections of the rostral portion of the shell of 

 Allorhynchus heteropsis (X 2^), showing the presence of dental lamellae in 

 the pedicle valve and the entire absence of lamellae in the brachial valve. 



Brachial valve much more convex than the pedicle, its greatest convex- 

 ity at or near the front margin, the surface slopes along the median 

 portion of the shell from the front margin to the beak with a regular or 

 with an increasing curvature, at the umbo the surface is impressed along 

 the median line in a shallow but distinct sinus which continues to the 

 middle of the valve either as a distinct sinus or as a flattened surface, to 

 the postero-lateral margns the surface slopes first with a convex curvature 

 and then becomes flattened or sometimes even concave; the mesial fold 

 is scarcely distinct back of the middle of the valve but anteriorly it is 

 rather strongly and abruptly elevated above the lateral slopes; the 

 lateral slopes are convex with the surface curving rather strongly both 

 anteriorly and laterally; the beak strongly incurved beneath that of 

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

 of the opposite valve. Internally the hinge-plate is divided to its ba'se 

 and lacks supporting septa of any kind, the cardinal process and median 

 septum wanting. 



Remarfa. In its usual condition of preservation the surface of this 

 species is exfoliated so that all indication of minute surface markings is 

 destroyed, but upon remnants of the shell which are sometimes present 



