BBACHIOPODA. 447 



ortliis ili.-ilnianella) subaequata.l 



are reduced in size and commonly open again near the anterior margin. On each side 

 of the median area the striae are finer, more numerous and rarely tubulose. Income 

 specimens, however, the perforations are more prolific near the cardinal margin. In 

 the dorsal valve the arrangement of the larger and smaller striae is the reverse of 

 that in the other valve. The finer, less tubulose ones are developed medially, while 

 those perforated and larger are situated laterally. Concentric lines excessively fine, 

 with a few coarser lines of growth near the anterior margin. Shell structure finely 

 punctate. 



Ventral valve strongly and evenly convex, usually with a broad, shallow sinus; 

 anterior margin slightly prolonged upward to correspond with the concavity in the 

 dorsal valve; point of greatest elevation about mid-length, or somewhat posterior; 

 apex greatly depressed. Cardinal area well developed, broadly triangular, elevated, 

 slightly convex and striated; delthyrium triangular, with straight or slightly convex 

 sides about twice as long as wide, the apex being occupied by a short plate or slightly 

 filled with shell matter. Interior with strong dental processes, supported by well 

 developed plates, which join the more or less strongly elevated postero-lateral mar- 

 gins of the muscular impression. Muscular area elongate, lobate, distinctly limited 

 by a raised margin in thick shells, while in thin ones the anterior edge may be obso- 

 lete, the center being occupied by the elongate adductors. The latter are medially 

 divided by a linear ridge, which in some specimens is well developed, separating 

 the scars in front and continuing as a strong septum to near the anterior margin. 

 Outside of the adductor scars are situated the diductors and adjusters, the latter 

 being well defined only in thick shells; pedicle muscle located in the apex of the 

 delthyrium, which may be either a thickening in that part of the shell or a short, 

 transverse, flat plate, similar to a delthyrium, with its anterior portion inwardly 

 directed and transversely striated. Genital spaces indicated on each side of the 

 musular area in thick shells. In such specimens the anterior margin is slightly 

 thickened and grooved, while in thin valves the entire interior is faintly striated, 

 conforming with the striae of the outer surface. 



Dorsal valve evenly convex, but not as deep as the ventral. In some specimens 

 there is a shallow and narrow medial depression, which has its origin immediately 

 below the beak and becomes obsolete near the mid-length or shortly anterior to it. 

 Cardinal area narrow and concave; delthyrium triangular, as wide as long, with a 

 linear elevation along each wall and centrally occupied by a small, faintly bilobed, 

 sometimes striated, cardinal process, which is anteriorly drawn out into a short, 

 slender septum. Crural processes strong and curving upward; with deep dental 

 sockets situated postero-laterally. The former are supported by strong plates, 

 which also form the walls of the delthyrium and are attached to the bottom of the 



