BRACHYTHYRIS 373 



Pedicle valve strongly convex, the greatest convexity near or a little 

 posterior to the middle, the umbonal region small, the surface curving 

 rather abruptly both anteriorly, laterally and posteriorly from the point 

 of greatest convexity; the beak small, rather short, incurved; cardinal 

 area small and low, arched, the lateral margins usually not discernable, 

 the surface of the area curving without demarkation into the lateral 

 slopes of the valve, occasionally the lateral margins may be detected as a 

 pair of faint ridges which slope from the beak to the cardinal extremities ; 

 the delthyrium broader than high, bordered on each side by a distinctly 

 elevated ridge which passes from the apex to the hinge-line ; lateral slopes 

 strongly convex, as the surface approaches the lateral and anterior mar- 

 gins the curvature becomes more abrupt, until in adult shells it is nearly 

 vertical to the plane of the valve, each marked by about 7 broad, de- 

 pressed, rounded, more or less obscure, simple plications; mesial sinus 

 narrow, shallow, rounded in the bottom, smooth, originating at the beak 

 and slightly produced in front in a nasute extension nearly at right angles 

 to the plane of the valve. 



Brachial valve less convex than the pedicle, the greatest convexity 

 near the middle, the surface curving more abruptly to the cardinal mar- 

 gin, sometimes a little depressed towards the cardinal extremities ; mesial 

 fold originating at the beak, smooth, rounded, but little elevated above the 

 general surface posteriorly and only moderately elevated towards the 

 front ; lateral slopes marked by plications similar in form and number to 

 those of the opposite valve, but somewhat stronger. 



The minute surface markings are not clearly preserved on any of 

 the specimens observed, but there is no indication of anything except 

 concentric markings. More or less inconspicuous concentric growth 

 lines are sometimes present. 



Remarks. This species is especially characterized by its rotund form. 

 It differs from 8. chouteauensis in its smaller size, greater convexity 

 of the valves, smaller and less-defined cardinal area, the less extended 

 beak of its pedicle valve, and the perfectly smooth fold and sinus. The 

 largest example observed has a length of 15 mm. 



Horizon. Fern Glen formation of the Kinderhook. 



BRACHYTHYRIS CHOUTEAUENSIS Weller 

 Plate LVII, Figs. 4-11 



1909. Spirifer chwtteauensis Weller, Bull. Geol. Soc. Am., vol. 20, p. 305, 

 pi. 13, fig. 11. 



Description. Shell suborbicular in outline, the length and width nearly 

 equal, hinge-line about two-thirds the greatest width of the shell, cardinal 

 extremities rounded. The dimensions of a nearly complete example are : 

 length of pedicle valve 22 mm., length of brachial valve 20 mm., length 



