68 MISSISSIPPIAN BRACHIOPODA 



Remarks. This species has been established upon two specimens, one 

 pedicle and one brachial valve. Both are from the Burlington limestone 

 of Henderson County, Illinois, and were apparently found in association, 

 although they are not the opposite valves of a single specimen. Both 

 valves are imperfect in several particulars, although their outlines are 

 well shown, and the brachial valve has been considerably crushed. In 

 neither specimen are the internal characters shown except very im- 

 perfectly, so it cannot be stated with absolute certainty that no median 

 septum is present in the pedicle valve, but the shell is exfoliated and par- 

 tially removed in the umbonal region of the specimen and there seems to 

 be no such structure present. In this specimen the muscular scar is 

 partially exposed; it seems to be large and flabellate, reaching beyond 

 the middle of the valve. The marginal convexity of the pedicle valve, 

 which has been described, is doubtless due to old age of the specimen, 

 and in younger examples the surface would doubtless be concave from 

 the beak to the front margin. The species is somewhat similar to the 

 one which has been here described as S. chouteauensis, but it is very much 

 larger besides having its greatest width along the hinge-line, having 

 subauriculate cardinal extremities. 



Horizon. Burlington limestone. 



Genus STREPTORHYNCHUS King 



Description. Shell biconvex, usually subglobose in form, the pedicle 

 valve or both valves more or less distorted and irregular in manner 

 of growth, the hinge-line shorter than the greatest width of the shell. 

 Cardinal area of the pedicle valve variable in height, differentiated into 

 a primary and a secondary area as in Schuchertella, the delthyrium closed 

 to its apex with the deltidium. Internally the pedicle valve is nonseptate, 

 the dental lamellae are reduced to ridges which are merely thickenings of 

 the inner surface of the cardinal area at the sides of the delthyrium, the 

 condition being essentially as in Schuchertella; in the brachial valve the 

 cardinal process is elongate and deeply grooved, its length being depend- 

 ant upon the convexity of the umbonal region of the pedicle valve. 



Remarks. The genus Streptorhynchus, as interpreted by Hall and 

 Clarke, 1 Girty, 2 and Thomas, 3 based upon S. pelargonatus Schl., as the 

 genotype, is characterized as follows: 1, the general form of the shell; 

 2, the absence of true dental lamellae ; 3, the absence of a median septum 

 in the pedicle valve; 4, the elongate, deeply grooved cardinal process. 

 From Orthotetes the genus differs conspicuously in the absence of the 



1 Pal. N. Y., vol. 8, pt. 1, p. 267 (1894). 



2 The Gaudalupian Fauna, p. 175 (1908). 



3 Mem. Geol. Surv. Gt. Brit., vol. 1, pt. 2, p. 89 (1910). 



