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



regions and curves more strongly to the hinge line on the posterior por- 

 tion ; no cardinal area ; beak small, not much elevated, incurved. 



Dorsal valve about equal in convexity to the ventral, somewhat elevated 

 from beak to front along the median line, forming a low, poorly denned 

 mesial fold, away from which the surface curves gently toward the front 

 and sides and more strongly toward the cardinal margin ; beak small and 

 inconspicuous, incurved beneath that of ventral valve. 



Surface of both valves marked by numerous indistinct, very fine lines 

 of growth ; without distinct radiating markings, but the fibrous structure 

 of the shell, where partly exfoliated, sometimes gives the appearance of 

 very fine radiating striae. 



Eemarks: This species differs from 8. davidsoni in its much shorter 

 length and proportionally greater width and the absence of such a promi- 

 nent anterior extension; from S. lirata, which it resembles in general 

 outline, it differs in the absence of distinct radiating markings. 



Horizon : Kankakee limestone at Oswego, Illinois. 



StricMandinia breviuscida n. sp. 

 Plate 16, Figure 5 



Description : Shell subcircular in outline, a little wider than long, the 

 greatest width slightly posterior to the middle; the lateral and anterior 

 margins rounded ; the .valves nearly equal in size and convexity ; hinge 

 line straight, equalling about half the greatest width of the shell. The 

 dimensions are : length, % inch ; greatest width, % inch ; thickness, nearly 

 y 2 inch. 



Ventral valve moderately convex, the greatest convexity at or posterior 

 to the middle, the median portion depressed longitudinally into a rather 

 broad shallow sinus. Cardinal area very narrow; beak low; spondylium 

 rather short and supporting median septum very short. 



Dorsal valve similar in form and convexity to the ventral, the median 

 portion elevated from beak to front into an indistinct mesial fold; the 

 surface is very gently convex or nearly flat on each side of the mesial fold 

 in the anterior portion, with a stronger curvature over the cardinal and 

 postero-lateral margins ; beak small and somewhat incurved. 



Surface with a few lines of growth near the margins, but without 

 radiating markings. 



Eemarks : This species differs from S. brevis in its smaller size and in 

 the stronger longitudinal convexity of the dorsal valve. It may prove to 

 be a young form of S. circularis, which it resembles in general outline, 

 but it differs from the normal shells of that species in the much smaller 



