72 Trans. Acad. Sci. of St. Louis. 



depressed; the cardinal process, as shown in casts of the 

 inside of the valve, is small, bifid, and lies in the plane of the 

 valve projecting beyond the hinge-line ; the internal casts also 

 show a low median ridge extending from near the beak almost 

 to the middle of the shell ; there are also two low, widely diverg- 

 ing socket plates whose impressions are preserved in casts of 

 the brachial valve. The surface of the pedicle valve is cov- 

 ered with innumerable fine spines which attain a length of at 

 least 5 mm. The brachial valve also is marked by small pits 

 which may represent a covering of spines, though the spines 

 themselves have not been observed. Both valves are marked 

 by more or less conspicuous concentric lines of growth. The 

 dimensions of the best preserved pedicle valve observed, one 

 of the type specimens, are : length, 20 mm., breadth, 22 mm., 

 thickness, 3^ mm. The largest specimen preserved is a 

 brachial valve whose length is 22 mm. and breadth, 32 mm. 



Remarks. This species was referred to the genus Stropha^ 

 losia by Winchell in his original description, but it is un- 

 doubtedly a Productella. The fine spines covering the surface 

 are usually destroyed, leaving only a pitted surface, but in a 

 single specimen of a pedicle valve in the University of Chicago 

 collection, their presence is clearly shown. The species 

 resembles in some degree P. pyxidata from the Louisiana 

 Limestone at Louisiana, Missouri, but it is a more nearly 

 circular shell and often attains a much greater size. 



PuGNAX STRiATOCOSTATA (M. and W.) var.? 



PI. II. f. 16-17. 



Shell much wider than long, broadly subtriangular in out- 

 line, the angle of divergence of the lateral margins from the 

 beak 110° to 113°. Pedicle valve depressed convex with a 

 broad, deep, sharply defined mesial sinus which is greatly 

 produced anteriorly in a lingual extension nearly at right 

 angles to the plane of the valve. Brachial valve extremely 

 gibbous, the mesial fold strongly elevated in front. Surface 

 of the shell marked by simple, strong, subangular plications, 

 of which there are usually four on each side of the mesial 

 sinus in the pedicle valve, and three on each side of the fold 

 of the brachial valve ; in the fold and sinus the number of 



