400 MISSISSIPPIAN BRACHIOPODA 



vex but is much more abrupt ; mesial sinus non-plicate, originating at the 

 beak, it is narrow and rather sharply defined in the umbonal region, be- 

 coming broader and less sharply defined anteriorly, deep and rounded in 

 the bottom, produced in front in a rounded lingual extension ; beak ob- 

 tusely pointed, usually nearly erect but sometimes a little incurved; car- 

 dinal area high and usually nearly flat, sometimes moderately arched 

 towards the beak, the entire surface in those specimens with a flat area, 

 and the lower, flatter portion in those becoming concave towards the beak, 

 slopes anteriorly from the hinge-line at an average angle of 58 degrees in 

 12 specimens, the lateral margins sharply defined, usually sloping from 

 the beak to the cardinal extremities with a gently convex curvature which 

 becomes a little more abrupt as it approaches the cardinal extremities ; 

 delthyrium large, broadly triangular, higher than wide, covered by a 

 pseudodeltidium which is usually destroyed ; each lateral slope marked 

 by from 18 to 24 simple, depressed, rounded plications which originate 

 along the cardinal margin. Internally the dental plates are short and 

 rather thick, the continuous portion of the transverse syrinx-bearing 

 plate connecting them is flat externally and extends one-third or less than 

 one-third of the distance from the beak to the hinge-line, with the free, 

 median, spine-like, tubular projection sharply differentiated from the 

 apical portion, more or less trifid at its termination, and reaching to a 

 point beyond the middle of the distance from the beak to the hinge-line ; 

 muscular scars strong and well-defined, subovate in outline, the adductor 

 scars narrow and elongate, occupying a rather sharply depressed area 

 along the median line of the valve, the diductor scars on either side 

 much larger and marked by numerous branching grooves and ridges 

 which tend to radiate in all directions from the lateral margins of the 

 adductor scars. 



Brachial valve strongly convex, its depth sometimes nearly or quite 

 equal to that of the pedicle valve, the surface convex from the cardinal 

 to the antero-lateral and anterior margins, with the curvature more ab- 

 rupt to the cardinal margin, towards the cardinal extremities the surface 

 usually becomes somewhat compressed; the rounded, non-plicate mesial 

 sinus is depressed towards the beak, becoming much higher and less 

 sharply defined anteriorly ; cardinal area narrow and inconspicuous ; beak 

 short, incurved, but little extended beyond the cardinal margin ; the pli- 

 cations of the lateral slopes entirely similar to those of the opposite valve 

 and alternate with them. Internally the cardinal process is broad and 

 flat, with a short median thickening of the interior of the valve in front 

 of its base, its surface of muscular attachment marked by numerous, fine, 

 vertical, angular grooves and ridges; the crural plates are thick and 

 strong, with rather small dental sockets excavated in their outer margins ; 

 the muscular scars large, reaching more than half-way to the anterior 



