72 The Saint Peter Sandstone. 



but flattened anteriorally and is continued in a rounded umbonal ridge 

 towards the dorso-ventral margin and in a broad depressed area towards 

 the ventral margin. The posterior margin is broadly rounded and is con- 

 tinuous with the ventral margin, which in turn continues with the anterior 

 forming a sigmoid curve. The surface of the cast is marked by fine con- 

 centric lines. All the specimens collected are imperfect and none show the 

 anterior perfectly. The specimen figured is from the transition strata of 

 the Saint Peter about four miles below Dodgeville, Wisconsin, along the 

 Illinois Central railway tracks. 



Modiolopsis litoralis n. sp. 

 Plate TI, figures 13, 14, 15 and 16. 



The figure represents an interior cast of average size and form of this 

 species. The casts are all strongly convex between the umbones and the 

 lower posterior margin, forming a broad umbonal ridge, and a broad 

 sinus, as well as a straightening of the posterior margin. The beaks are 

 indistinct but extend longitudinally and project above the hinge. One 

 strong anterior muscle scar is situated apparently just at the anterior end 

 of the hinge. Compare Orthodesma curvatum H. and W. Pal. O. ii, p. 95. 



From South Saint Paul and Highland Park. 



Modiolopsis contigua n. sp. 

 Plate II, figure 8. 



Shell small, length about 25 mm., bredth at the posterior extremity of 

 the hinge, 15 mm., at the umbones about 9 mm. The hinge is somewhat 

 curved and is in some specimens longer posteriorally than in the one fig- 

 ured, and in these it forms an obtusely rounded angle with the posterior 

 margin. The ventral margin is concave or straight, the anterior margin 

 short and strongly convex. The beaks are somewhat obtuse and are di- 

 rected forward. Their posterior half swells into a high rounded umborial 

 ridge that extends towards the posterior- ventral margin while the anterior 

 half is flattened continuous with the concave area anterior to the umborial 

 ridge. The greatest convexity is at one-third the distance from the um- 

 bones towards the lower posterior margin. The shell has been very thin 

 and marked with growth lines. It differs from M. plana Hall, in being pro- 

 portionately narrower and having a stronger umbonal ridge and more 

 concave ventral margin. From M. similis Ulrich it differs by the same 

 characters. The type specimen is a relatively broad one. 



Found at South Saint Paul. 



Modiolopsis gregahs n. sp. 

 Plate II, figures 10, 11 and 12. 

 Length about 25 mm.; bredth 12 mm. and greatest at the middle; 

 beaks less than one fifth the distance from the anterior end, small, incurved 

 and scarcely projecting. Hinge line nearly straight ; dorsal margin rounded 

 gently from the hinge but more strongly so near the ventral margin ; 

 ventral margin gently arcuate and gradually rounding up continuous with 



