18 Trans. Acad, Set. of St. Louis. 



the front of the shell and occupied by bifurcating plications 

 which are about six in number at the front margin. Bra- 

 chial valve regularly convex except for a slight compression 

 towards the cardinal extremities, the greatest convexity at a 

 point on the median line between the beak and the middle 

 of the shell ; the mesial fold but slightly elevated above the 

 general level of the valve, marked by two plications at the 

 beak which bifurcate and increase to from four to six at the 

 anterior margin. Surface of each valve marked by from 

 fifteen to twenty-two rounded plications upon each side of 

 the fold and sinus, with a few of those nearest the fold and 

 sinus sometimes bifurcating in an irregular manner. Aside 

 from the plications the shell is marked by fine concentric 

 lamellose striae or lines of growth. 



Remarks. This species exhibits a considerable range of 

 variation in some of its characters. The largest specimens 

 attain a width along the hinge-line of over 45 mm. while the 

 smaller ones are scarcely 20 mm. in width. There is also 

 great variation in the coarseness of the plications, and in the 

 bifurcation of the first ^\q or six on either side of the fold 

 and sinus. In some individuals these plications are entirely 

 simple, while in others they bifurcate at almost any point 

 between the beak and the front margin. There is also a 

 variation in the cardinal extremities, some of them being 

 almost rectangular, while in others they are produced into 

 mucronate points. The most reliable character for the identi- 

 fication of the species is the low mesial fold of the brachial 

 valve, and the narrow area with subparallel sides. In some 

 specimens there is almost no elevation of the fold above the 

 general level of the shell, the fold being bounded by two 

 furrows a little deeper than the others, while in other indi- 

 viduals the fold has a moderate elevation. 



SpIRIFER. Cf . S. STRIATIFORMIS Mcck. 



A few specimens of a species of Spirifer different from 

 S. marionensis, have been found in the Northview fauna. 

 Only pedicle valves have been observed and none of these are 

 sufficiently well preserved to allow of a full description. They 

 differ from jS. marionensis in being larger than the average 



