334 Lower Silurian Faima of Minnesota. 



SKENIDIUM ANTHONENSIS n. Sp. 



Plate IV, figure 7. 



Very small, length of ventral valve 3 mm. or less, length of dor- 

 sal 2 mm., breadth 5 mm. Area mostly on the ventral valve, large, 

 plane, nearly equal in dimension to the dorsal valve and at right 

 angles to it. Deltidium very large, extending to the beak on both 

 valves. Surface of the ventral valve plicated by 26 simple radise, two 

 pair of which arise from the beak and include one shorter pair be- 

 tween them. The others arise from between the beak and the ex- 

 tremities of the area. Beak acute, not produced. A median fold extend- 

 ing from the beak to the mesial line includes about six radiae, is 

 nicely rounded, and has a corresponding sinus on the dorsal valve, 

 and an emargination of the mesial line. Plications on the dorsal same 

 as on the ventral surface. 



From the Buff limestone, near the Falls of Saint Anthony at Min- 

 neapolis, associated with Zygospira aquila n. sp. 



STROPHOMENA HALLI n. Sp. 



Plate IV, figures 36, 37 and 38. 



Shell delicate, semicircular, width about 25 mm., length 15 mm. 

 Ventral valve strongly convex near the beak, concave thence to the 

 umbo where it suddenly turns and curves up to the mesial line. Curv- 

 ature of the dorsal valve follows that of the ventral. Surface of both 

 valves marked by fine radiating lines with about 25 or 30 heavier ones. 

 The concave part of the ventral valve and corresponding convex part 

 of the dorsal valve are undulated somewhat like confused cross waves. 

 This marking extends to the interior, i e. the whole shell undulates. 

 No concentric lines common. Area on the ventral valve unusually 

 large for a strophomena, and has a large triangular deltidium. 



Named in honor of Prof. C. W. Hall of the State University, to 

 whose collection the first specimens seen belong. The specimens fig- 

 ured are from the Stictopora bed, Saint Paul. Rare. 



STROPHOMENA INQUASSA n. Sp. 



Plate V, figures 22, 22, and 24. 



Shell subcircular, length 25 mm., breadth 30 mm., hinge line equal 

 to or less than the greatest width. Ventral valve strongly and evenly 

 convex; (shells that are injured during growth, bend abruptly back at 

 the line of injury, and the slope from thence to the mesial line is di- 

 rect). Dorsal valve deeply concave. Area on the ventral valve broad; 

 deltidium large. Surface radially marked like S. allernata Conrad. 

 On the ventral side between the beak and each cardinal angle are from 

 five to eight short, indistinct folds, oblique with reference to the med- 

 ian line. 



The shell is thick and the internal markings are heavy. In these 

 two respects as well as in having a shorter, broader hinge area and a 

 more convex valve, it differs from S. altcrnata Conrad., to which it is 



