186 



MISSISSIPPIAN BRACHIOPODA 



surface anteriorly lies in nearly a right angle to the plane of the valve; 

 beak rather blunt and short, incurved and nearly in contact with the 

 urabo of the opposite valve, the delthyrium apparently broadly triangular 

 and nearly filled by the beak of the opposite valve, the deltidial plates 

 and foramen obscure and not observed in any of the specimens studied; 

 plications almost obsolete, limited to from two to four, all of which are 

 present in the anterior part of the sinus, having their origin near or in 

 front of the middle of the valve. Internally the hinge-teeth are sup- 

 ported by a pair of short dental plates which scarcely extend beyond the 

 articulation of the hinge., 



O 



o 



FIG. 7. A series of seven cross-sections of the rostral portion of the shell of 

 Liorhynchus greenianum (X 2%), showing the dental lamellae in the pedicle 

 valve and the median septum and crural cavity in the brachial valve. 



Braehial valve gibbous, its convexity much greater than that of the 

 pedicle valve, the surface strongly arched from beak to front along the 

 median line with an increasing curvature towards the beak, laterally from 

 the median line the surface curves abruptly to the margins, the curvature 

 usually being a little broken towards the front by the differentiation of 

 the mesial fold ; the mesial fold obsolete in the posterior half of the valve, 

 usually but not always slightly differentiated anteriorly; the beak 

 strongly incurved beneath that of the opposite valve; the plications 

 similar to those of the opposite valve and restricted to the anterior portion 

 of the mesial fold. Internally the cardinal process is wanting and the 

 hinge-plate is divided ; at the apex of the valve a divided median septum 

 is present, one side of which supports each side of the divided hinge-plate 

 at its initial point, the space between the two divisions forming a crural 

 cavity which, however, is very short and terminates before the articula- 

 tion of the hinge is reached, beyond the crural cavity the septum is rapidly 

 reduced in height and continues anteriorly for from one-fourth to one- 

 third of the length of the valve as a faint ridge. 



