374 MISSISSIPPIAN BRACHIOPODA 



of hinge-line 13.5 mm., greatest width of shell, 24.5 mm., thickness 17 

 mm., height of cardinal area 2 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, the umbo rather narrow, prominent, the surface curving abruptly 

 from the umbonal region to the cardinal margin and more gently to the 

 antero-lateral margins, sometimes a little compressed towards the car- 

 dinal extremities ; beak small, sharply pointed, rather strongly incurved ; 

 cardinal area small, arched, the curvature becoming greater towards the 

 beak, lateral margins more or less ill-defined, sloping from the beak to 

 the cardinal extremities ; lateral slopes convex antero-posteriorly, marked 

 by from 8 to 11 simple, rounded plications ; mesial sinus narrow, rather 

 shallow, rounded in the bottom, well-defined to the beak posteriorly, 

 becoming less well defined anteriorly, bounded by a pair of plications 

 which are notably broader and stronger than those of the lateral slopes, 

 marked by two or four obscure, depressed, rounded plications which 

 originate from the inner margins of the bounding plications. 



Brachial valve less convex than the pedicle, the greatest convexity 

 near the middle, the surface curving abruptly to the cardinal margin, 

 and more gently to the antero-lateral margins, usually somewhat com- 

 pressed towards the cardinal extremities; mesial fold defined to the 

 beak, convex, scarcely elevated above the general surface posteriorly, 

 moderately elevated in front, obscurely marked by plications similar 

 in form and number to those of the opposite valve. 



The minute surface markings of the shell consist of obscure, exceed- 

 ingly fine, concentric stria? which are ordinarily obliterated in the fossils. 

 At irregular intervals several concentric lines of growth are usually 

 present. 



Remarks. The plication of the fold and sinus of this species is very 

 obscure, and not infrequently seems to be absent altogether at first 

 sight. In its general form and proportions the species resembles S. 

 suborbicularis, but it is always much smaller than the adult examples of 

 that species. The cardinal area of the pedicle valve in S. chouteauensis, 

 however, is proportionately smaller and less well defined than in S. 

 suborbicularis. Members of this species have not infrequently been identi- 

 fied as S. peculiaris, but it differs conspicuously from that species in the 

 lower and more defined cardinal area, the more approximate beaks of 

 the two valves and in the obscure plications of the fold and sinus. 



Horizon. Kinderhook. 



BRACHYTHYRIS SUBORBICULARIS (Hall) 

 Plate LXI, Figs. 1-8 ; Plate LXII, Pigs. 1-12 



1858. Spirifer suborbicularis Hall, Geol. Iowa, vol. 1, pt. 2, p. 644. 

 1875. Spirifer sub orbicular is Meek and Worthen, Geol. Surv. 111., vol. 6, 

 p. 523, pi. 30, fig. 1. 



