580 PALEONTOLOGY OF ILLINOIS. 



side short, rounding from above the middle into the base ; 

 outline of ventral margin forming a broad semi-ovate 

 curve ; posterior margin nearly vertically subtruncated, 

 but rounding abruptly into the dorsal margin above and 

 into the base below ; cardinal border nearly straight, and 

 sloping very slightly from the beaks posteriorly; beaks 

 much depressed, nearly anterior, incurved, closely approxi- 

 mated, and directed forward and inward. Posterior ab- 

 ductor muscular scar shallow, rather large, suboval, and 

 located close up under the posterior extremity of the hinge ; 

 anterior do., smaller, deeper, subovate, and placed very near 

 the anterior margin a little above the middle, with rather 

 distinct, nearly detached pedal muscular scars at their up- 

 per ends. Posterior umbonal slopes with each a distinct 

 sulcus extending from the beaks obliquely backward and 

 downward, becoming wider and more shallow as they de- 

 scend, so as to die out before reaching the posterior basal 

 margin. (External surface unknown.) Length of cast, 

 2.60 inches ; hight of do., 2.15 inches ; convexity, 1.56 

 inches. 



This is the largest species of ScMzodm (if it belongs to that genus) we 

 have yet seen. Our only specimen is an internal cast, giving no idea 

 of the nature of the hinge. From its general appearance, however, 

 and the nature of its muscular and simple pallial scars, together with 

 the remains of its external ligament, we can have little doubt in regard 

 to its being a Sclrizodus. In size and general outline its internal cast 

 closely resembles AmpMccelia Leidyi, described by Prof. HALL, from 

 the Upper Silurian Limestone at Bridgeport, near Chicago. It has, 

 however, obviously no near relations to that shell. For a tichizod'ux its 

 beaks are unusually depressed and oblique. We know of no other spe- 

 cies resembling it in general form. 



Compared with ScMzodus occidental fCyprioardiaf occidcntaU-Sj 

 Swallow J which seems to be a true Schizodux, and agrees with our 

 shell in size, it is at once distinguished by its much more depressed 

 beaks, less convex valves, and broadly round posterior outline. Prof. 

 SWALLOW'S species also occurs in our Coal .Measures, but we have only 

 yet seen it from a higher part of the series at La Salle. 



Position and locality Seaville, Fulton county, Illinois ; roof of coal 

 No. 1. 



