318 T. E. SAVAGE ALEXANDRIAN ROCKS OF ILLINOIS AND WISCONSIN 



becomes flattened in the more narrowed portion of the shell ; the cardinal 

 area is narrow, concealed when the valves are in place; beak very low 

 and inconspicuous, incurved. 



The dorsal valve nearly as convex as the ventral, gently curving longi- 

 tudinally from beak to front ; highest along the median line, but without 

 a distinct fold; transversely, it is rather strongly convex, the curvature 

 more abrupt toward the postero-lateral margins ; sometimes with a slight 

 depression near the front on each side of the median portion ; beak small 

 and incurved beneath that of the ventral valve. 



Surface of both valves marked by numerous lines of growth, some of 

 which are stronger than others, but without definite radiating markings. 

 The dimensions differ greatly in different specimens. Those of about an 

 average shell are: length, 2% inches; greatest width, 1% inches; thick- 

 ness, 1 inch. 



Eemarks: This form somewhat resembles 8. davidsoni Billings in its 

 generally large size, elongate form, and the short hinge line in proportion 

 to the greatest width of the shell. It differs from Billings' species in 

 being larger and thicker, in having a much shorter hinge line, in the 

 widest part of the shell being at or anterior to the middle, and in the 

 flattened postero-lateral margins. In some respects it also resembles 8. 

 deformis Meek and Worthen, from which it differs in the relatively 

 shorter hinge line, the more symmetrical growth, the shell showing no 

 strong concentric constrictions, and in the less oblong outline, the poste- 

 rior half of the shell being relatively much narrower than in S. deformis. 



Horizon : About 18 to 25 feet above the base of the Kankakee limestone 

 at Hamburg, along Kankakee Biver 5 miles south of Bichey, and at Elgin, 

 Illinois. 



Stricklandinia pyriformls var. elongata n. var. 



Plate 16, Figure 7 



Description : The shells of this variety differ from those of 8. pyri- 

 formis chiefly in being much thinner, in having the hinge line nearly or 

 quite as long as the greatest width of the shell, and the cardinal extremi- 

 ties more or less produced, so as to make the cardino-lateral angles slightly 

 less than right angles. The shells are somewhat oblong in outline, a little 

 longer than wide, about equally and very moderately convex, with greatest 

 elevation along the median line, but without definite fold or sinus on 

 either valve; the sides are nearly parallel, and the front margin regularly 

 rounded ; cardinal areas linear and beaks obscure. 



The dimensions of a cast are: length, 1% inches; width, iy 8 inches; 

 thickness, about ^2 inch. 



