488 MISSISSIPPIAN BRACHIOPODA 



tional width and thickness of the shells is also extremely variable. The 

 diminutive individuals from the Spergen Hill fauna are apt to be more 

 globular than the larger ones. The mesial furrow, present sometimes in 

 both fold and sinus, is a variable character, as are also the f urrows ( outside 

 the lateral margins of the sinus of the pedicle valve. 



A nearly related species is C. subquadrata from the Chester fauna, and 

 specimens here referred to C. trinuclea have frequently been called C. sub- 

 quadrata, in fact these two names have sometimes been considered as 

 synonyms. There seems to be a constant difference between the two 

 forms, however, in the more posterior extension of the mesial sinus of the 

 pedicle valve in C. trinuclea. 



Horizon. Salem limestone, St. Louis limestone, Ste. Genevieve lime- 

 stone, Chester group. 



COMPOSITA LEWISENSIS n. Sp. 



Plate LXXXI, Figs. 46-51 



Description. Shell small, ovate-subpentagonal in outline, longer than 

 wide, the greatest width anterior to the mid-length. The dimensions of 

 two nearly complete specimens are : length of pedicle valve 10.4 mm. and 

 10.6 mm., length of brachial valve 9.6 mm. and 9.6 mm., greatest width 

 9.4 mm. and 9.2 mm., thickness 6.8 mm. and 6.2 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly to the postero-lateral margins, becoming inflected to the car- 

 dinal extremities and curving much more gently to the antero-lateral and 

 anterior margins ; mesial sinus inconspicuous, usually originating near or 

 in front of the middle of the valve, very shallow, rounded in the bottom, 

 ill-defined at its lateral margins ; the beak small, pointed, rather strongly 

 incurved, perforated by a subcircular foramen which encroaches wholly 

 upon the umbonal region of the valve ; cardinal area obsolete, the delthy- 

 rium closed. 



Brachial valve subequally or a little less convex than the pedicle, the 

 greatest convexity posterior to the middle, the surface curving rather 

 abruptly to the postero-lateral margins and more gently to the antero- 

 lateral and anterior margins; mesial fold obsolete, the mesial portion of 

 the valve usually obscurely flattened anteriorly from the middle, and 

 sometimes very slightly depressed towards the front margin to form a 

 slight sinus which is fainter and more obscure than that of the opposite 

 valve ; beak pointed, strongly incurved beneath that of the opposite valve 

 and partially filling the delthyrium. 



Surface of both valves marked by somewhat regular, concentric lines 

 of growth which are sometimes nearly obsolete except near the anterior 

 margin. 



Remarks. This little shell is characterized by the absence of a mesial 

 elevation of the brachial valve, and by the slight mesial sinus of the ped- 



