Lower Silurian Fauna of Minnesota, 333 



mens in hand, but either there is a large area or the dorsal valve is a pen- 

 tagonal in outline. The surface is ornamented by pits arranged in con- 

 centric and radiating rows crowded on the posterior surface, alternating 

 with smooth concentric bands on the anterior slope. Casts of the interior 

 pitted likewise. The smooth bands are marked only by rounded growth 

 lines. 



Rare; in the crystalline strata at the bottom of the Stictoporella bed 

 at Minneapolis. 



RHYNCHONELLA MINNESOTENSIS n. Sp. 



Plate IV, figures 21, 22 and 23. 



Compare R. increhescens Hall and R. orientah's Billings. 



Shell variable in size, length of mature specimens 10 to 15 mm., 

 breadth 12 to 18 mm., depth usually about one-half of the length, but 

 specimens are often anteriorly globose. From 15 to 25 plications on the 

 ventral valve, one to eight of which may fall in the sinus but more often 

 there are three, four or five. Always one more on the fold of the dorsal 

 valve. Plications rarely bifurcate. Sinus extends about two-thirds the 

 distance to the beak. No elevated concentric striae. Beak moderately re- 

 curved as in R. increhescens Hall, as identified in the Fucoid bed atid up- 

 wards. There are undoubted transitional forms between this species and 

 R. increhescens, on both sides of the line between the Stictopora bed and 

 the Fucoid bed. But the greater breadth, greater curvature along either 

 valve from the beak to the opposite point on the mesial line, broader or 

 less acute radiae, lack of elevated concentric lines and less asperated np- 

 pearance readily distinguished R. minnesotensis. 



In relation to R orientalis Billings, identified in the Buff limestone, 

 there are many apparent intermediate forms recurring among R. minne- 

 sotensis, but the deeper sinus, larger size and more numerous plications 

 distinguish the last named species. 



Found everywhere in Minnesota in the Blue limestone and to the top 

 of the Stictopora bed. 



RHYNCHONELLA SANCTA n. Sp. 



Plate IV, figures 19 and 20. 



Shell transversely elliptical, depressed, length 13 mm,, width 17 mm., 

 depth 7 mm. Thirteen broad radiating plications on the ventral valve, and 

 fourteen on the dorsal. Sinus on the ventral valve, contains three plica- 

 tions; four plications on the fold on the dorsal valve. Beak curved. 

 From the umbo on the dorsal valve, along the fold to the mesial line, 

 scarcely curved. No concentric lines except one or two growth lines which 

 seem to be accidental. Intermediate forms between this species and R. 

 increhescens Hall, with which it is associated, are rather more numerous 

 than specimens of R. sancta, but the striking differences and wide distri- 

 bution of both, make it seem improbable that they are even varieties of 

 the same species. 



From the Orthisina ( ?) bed at Saint Paul and in Goodhue county. 



