670 



THE PALEONTOLOGY OE MINlirESOTA. 



[Drepanella. 



This species, though smaller and distinct, resembles the Devonian B. ungula 

 Jones, more closely than any other of the genus known. B. persulcata of the 

 Cincinnati rocks is perhaps as near as any of the Silurian forms, but there are so 

 many differences between them that it is unnecessary to enter into comparisons. 

 In B. subcequata, which belongs to another section of the genus, both the inner and 

 outer ridges are much thinner. 



Formation and iocaHf^/.— Associated with the preceding in the Galena shales, in Goodhue county, 

 Minnesota. 



Genus DREPANELLA, Ulrich. 



Drepanella (Depranella in error) Ulrich, 1890, Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 117. 



Carapace equivalved, compressed-convex, somewhat oblong, the outline between 

 subquadrate and subelliptical; dorsal border straight, ventral outline gently convex; 

 ends subequal, the posterior somewhat truncated above, the anterior generally more 

 rounded. Running nearly parallel with and close to the posterior and ventral edges, 

 a sharply elevated, sickle-shaped ridge. Central and dorsal regions of valves with 

 two principal, simple or divided, nodes or ridges. Surface smooth or reticulate. 

 Size of carapace usually about 2.5 mm. long by 1.5 mm. high. 



Type: D. crassinoda Ulrich, 



Fig. 48.— a, 6, lateral and dorsal views of a right valve of Drepanella crassinoda Ulrich, from the 

 Birdseye limestone at High Bridge, Kentucky; c, a left valve of D. ampla, var. elongata Ulrich, from the 

 same formation and locality, introduced for comparison with D. bigeneris; d, right valve of D. macro 

 Ulrich, from the same formation at Dixon, Illinois; all x 20. The last probably occurs also in Minnesota. 



Of this genus only Lower Silurian species are known. Taking the six species 

 and varieties upon which the genus was founded, we have a sharply defined generic 



