BEACHIOPODS. 65 



specimens, often becoming so gibbous as to assume a sub-glo- 

 bose form ; hinge-line usually a little less than the greatest 

 breadth of the valves but sometimes equaling, or somewhat ex- 

 ceeding, the same ; cardinal extremities more or less obtusely 

 angular, nearly rectangular, or sometimes rather sinuous behind 

 and rounding to the front, which is a little sinuous, rounded or 

 somewhat prominent at the middle ; beaks and cardinal areas 

 of the two valves nearly equal, the former incurved and ap- 

 proximate, or, in adult shells, sometimes contiguous. Dorsal 

 valve generally a little more convex than the other, in some 

 examples rather decidedly so (its greatest convexity being near 

 the middle), provided with a moderately prominent, rather 

 rounded mesial fold, that commences at or near the beak, and 

 continues forward, gradually widening and rounding over with 

 the curve of the valve to the front, where it is moderately ele- 

 vated, with more or less sloping sides ; lateral slopes convex ; 

 beak projecting beyond the hinge margin, strongly incurved, 

 particularly in old individuals, which sometimes have the gib- 

 bous umbo projecting even a little beyond that of the other 

 valve; cardinal area well developed, distinctly defined, nearly 

 as wide as that of the other valve, directed backward, and more 

 or less strongly incurved; foramen broad-triangular, and not 

 closed by the cardinal process. Interior showing the cardinal 

 process to be very small, or merely having the character of a 

 lew linear ridge in the rostral cavity ; posterior pair of muscu- 

 lar impressions corrugated, and decidedly larger and more 

 widely separated than the others. 



Ventral valve with a mesial sinus corresponding to the 

 fold in the other valve, and terminating at the front in a rather 

 short, somewhat rounded projection, that curves more or less 

 upward into a sinuosity of the same size and form in the 

 margin of the dorsal valve ; beak usually a little less strongly 

 incurved than that of the other, and very slightly more prom- 

 inent at its apex; cardinal area about one-fourth to one-third 

 higher at the middle than in the dorsal valve, and narrowing 

 less rapidly toward the lateral extremities ; incurved and di- 

 rected backward, but a little less strongly so than the other; 



