344 MISSISSIPPlAlsr BRACHIOPODA 



its height 3 to 4 mm., the inferior portion sloping posteriorly at an angle 

 of about 135 degrees to the plane of the valve, the lateral margins sharply 

 defined and curving gently from the beak, the curvature becoming more 

 abrupt as it approaches the cardinal extremities, the area marked by 

 vertical striae and when perfectly preserved by fine crenulations along the 

 cardinal margin ; lateral slopes of the valve each bearing from 14 to 18 

 rounded plications which are usually simple except that the second one 

 on each side originates from the outer side of the first one close up to the 

 beak, and occasionally the third one is given off from the inner side of 

 the fourth ; the mesial sinus originates at the beak where it is sharply 

 defined and angular, becoming rounder in the bottom anteriorly and less 

 strongly defined, marked by a median plication which originates near 

 the beak and continues to the anterior margin gradually increasing in size, 

 each lateral slope of the sinus marked by one plication, or occasionally 

 two, which originate from the inner margins of the lateral bounding 

 plications, the lateral plications are usually weaker than the median one 

 and when two are present the one which originates most anteriorly is 

 much fainter than the other. 



Brachial valve with its greatest convexity near the middle, the surface 

 sloping more abruptly towards the cardinal margin, usually somewhat, 

 compressed towards the cardinal extremities ; mesial fold sharply defined 

 at 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 ; it is marked by a median furrow corresponding with the 

 median plication of the sinus, and usually with two plications on each 

 side ; the lateral slopes are marked by plications similar in form and 

 number to those of the opposite valve. 



The surface of both valves marked by exceedingly fine concentric and 

 longitudinal lines which give it a finely reticulate appearance under a 

 lens, and by coarser lines of growth which are sometimes crowded 

 towards the anterior margin. 



Remarks. This species is most like 8. pellaensis, but is always larger in 

 its adult condition, with a greater number of plications, and does not 

 exhibit so great variation in its general outline, being more uniformly 

 extended along the hinge-line. The occasional division of one of the 

 larger plications upon the lateral slopes of the shell is also a feature 

 v^hich has not been observed in 8. pellaensis. Immature examples of 8. 

 increbescens, however, are almost identical in general .form with the 

 smaller species and may not possess a larger number of plications, but 

 they may usually be recognized .as immature by the lack of crowding of 

 the lines of growth towards the anterior margin. 



Horizon. Chester group. 



