76 MISSISSIPPIAN BRACHIOPODA 



of the delthyrium, the dental plates usually join the median septum near 

 the beak to form a shallow, triangular chamber in the apex of the valve; 

 the muscular impression is large and flabellate, triangularly subovate in 

 outline, not deeply impressed, and reaching nearly or quite to the middle 

 of the valve. 



Brachial valve strongly convex, the greatest convexity posterior to the 

 middle, the surface curving more abruptly to the cardinal margin and 

 less abruptly to the lateral and anterior margins, distinctly compressed 

 towards the cardinal extremities, the umbo projecting slightly posteriorly 

 beyond the hinge-line; median sinus obsolete, although the median por- 

 tion of the valve is frequently if not always slightly flattened. Internally 

 the cardinal process is strong, extending conspicuously beyond the car- 

 dinal margin, it is slightly bifid at the extremity and is produced ante- 

 riorly as a distinct, flattened, ridge-like thickening upon the inner surface 

 of the valve, which at first contracts in width from the base of the car- 

 dinal process and then expands anteriorly as it is gradually merged with 

 the general inner surface within the umbonal region of the valve; at the 

 base of the cardinal process the hinge sockets are excavated in the sides 

 of that process, the socket or crural plates being insignificant in their 

 development ; muscular scars ill-defined, often scarcely distinguishable. 



Surface of both valves marked by fine, subangular, radiating costae 

 which are somewhat irregular in size and increase by bifurcation and in- 

 tercalation, from 1 to 3 occupying the space of one millimeter. "When 

 the surface is uninjured the costae are crossed by exceedingly fine, con- 

 centric striaa, and by occasional, distinct, concentric, wrinkle-like lines of 

 growth. Upon the internal surface of the shell, as shown in internal 

 casts, concentric markings nearly equaling the radiating costae in size, but 

 somewhat more weakly developed, are commonly noticeable, which give 

 to the surface a distinctly reticulate appearance. 



Remarks. It has been shown by Girty 1 that the typical species of the 

 genus Orthotetes Fischer de Waldheim, possesses a median septum in the 

 pedicle valve, which is the essential character of the genus Derbya as de- 

 fined by Waagen, a condition which necessitates the abandonment of the 

 name Derbya and the substitution of a new generic name for the shells to 

 which Hall and Clarke have applied the name Orthotetes. There are 

 recognized among the septate shells of Waagen 's genus Derbya, two rather 

 distinct groups of species, in one of which the septate dental plates join 

 with the median septum and, with the delthyrium, enclose a triangular 

 chamber in the apical portion of the pedicle valve, while in the second 

 group of species no such triangular chamber is present. It happens that 

 the genotype of Orthotetes is a member of the first or camerate division 

 of the genus, while that of Derbya is one of the simply septate forms, so 



1U. S. Geol. Survey, Prof. Paper, No. 58, pp. 186-199 (1908). 



