216 MISSISSIPPIAN BRACHIOPODA 



anterior portion of the valve; beak acutely pointed and moderately in- 

 curved; plications sixteen to twenty in number, simple and reaching 

 from the beak to the margins without intercalations, rounded posteriorly 

 but becoming more or less subangular near the anterior and antero- 

 lateral margins, those upon the postero-lateral deflections of the valve 

 curving strongly towards the margin. 



Brachial valve much deeper than the pedicle, its greatest depth near 

 the anterior margin, gently convex along the median line from the point 

 of greatest depth to the beak and curving abruptly from the same point 

 to the anterior margin, gently convex transversely across the central 

 portion of the valve and abruptly deflected laterally towards the opposite 

 valve, postero-laterally the deflected portion is a little concave, becoming 

 a little convex anteriorly, the deflected portion being much broader than 

 the similar deflected portion of the opposite valve ; mesial fold not dif- 

 ferentiated from the general curvature of the valve except in the sinuosity 

 of the anterior line of junction of the two valves ; the beak pointed and 

 incurved beneath that of the opposite valve ; plications similar in form 

 and number to those of the opposite valve. 



Surface of both valves marked by fine, regular, concentric lines of 

 growth which follow the direction of the serrate margin, upon the older 

 portion of the shell they are commonly obliterated, even upon the best 

 preserved specimens, but towards the margin they are clearly defined 

 unless the surface is exfoliated, a few stronger lines of growth are usually 

 present at irregular intervals, or they may be entirely absent; besides 

 the concentric lines the surface is marked by exceedingly fine radiating 

 striaB which are best observed only upon the postero-lateral deflected 

 portions of the valves, and are entirely obliterated upon exfoliated shells. 



Remarks. The internal characters of representative examples" of this 

 species have not been carefully examined because of the lack of material, 

 but enough has been seen to show that these structures are essentially 

 identical with those of T. siibcuneata. Indeed the whole aspect of a 

 good example of the species is that of a rather large specimen of T. 

 subcuneata, and it is perhaps ill-advised to consider these two species 

 as distinct. If T. arctirostrata is a good species it must rest upon its 

 larger size, deeper brachial valve, and especially upon the fine radiating 

 costae. 



Horizon. Salem limestone. 



TETRACAMERA MISSOURIENSIS n. sp. 



Plate XXVIII, Figs. 35-39 



Description. Shell of medium size or usually somewhat smaller, trian- 

 gularly subovate in outline, the greatest width anterior to the middle, 

 usually longer than wide, the long postero-lateral margins nearly straight 



