BRACHYTHYRIS 377 



into the lateral slopes of the valve without any sharp line of demarca- 

 tion; lateral slopes of the valve each bearing from 8 to 10 broadly 

 rounded, simple plications which grow progressively smaller towards the 

 cardinal extremities; mesial sinus originating at the beak where it is very 

 narrow and well defined, anteriorly it becomes broad and shallow, flat or 

 roundish in the bottom, and in large examples it is sometimes produced 

 in a nasute extension anteriorly, it is usually occupied by three rounded 

 plications of nearly equal size, similar to the larger plications upon the 

 lateral slopes, all three plications originate near the beak, the two lateral 

 ones originating from the inner margins of the lateral bounding plica- 

 tions a little in front of the origin of the median one, sometimes the 

 number of plications is reduced to two in number and sometimes in- 

 creased to four, but often when the number is abnormal they are not so 

 nearly equal in size as in normal individuals. 



Brachial valve less convex than the pedicle, its greatest convexity near 

 the middle, the surface sometimes a little compressed towards the cardinal 

 extremities, the beak projecting a little posteriorly from the hinge-line; 

 mesial fold defined to the beak but scarcely elevated above the general 

 surface of the valve, becoming gradually elevated to the front where it is 

 rounded and of moderate height, in large individuals it is sometimes 

 rather abruptly elevated near the anterior margin, in normal specimens 

 it is marked by four, subequal, convex plications which originate from a 

 single one at the beak, in individuals with more or less than three plica- 

 tions in the sinus of the pedicle valve there is a like variation in the fold 

 of the brachial valve ; lateral slopes of the valve bearing plications similar 

 in form and number to those of the opposite valve. 



Minute surface markings of the shell not observed, but concentric 

 lines of growth of greater or less strength are usually present, which 

 are sometimes crowded towards the anterior margin in large individuals. 



Remarks. This shell is a very characteristic member of the Salem 

 fauna in Indiana and Illinois and has not been certainly seen from 

 any other horizon, although some specimens are labeled as coming from 

 the Keokuk limestone in Indiana. The shell is especially characterized 

 by its very narrow cardinal area with the false area-like regions on 

 either side of the true cardinal area. In general outline the shell resem- 

 bles 8. sub orbicular is, but the plications are stronger in 8. subcardiiformis, 

 especially those of the fold and sinus. 



Horizon. Salem limestone. 



BRACHYTHYRIS GURLEYI n. sp. 



Plate LV1II, Figs. 4-8 



Description. Shell of medium size, suborbicular in outline, the greatest 

 width near the mid-length of the shell, the cardinal extremities rounded. 



