668 THE PALEONTOLOGY OF MINNESOTA. 



[Jonesclla obseuni. 



JONESELLA OBSCURA, tt. Sp. 



PLATE XLIV, FIGS. 17-19. 



SIX.E. Length 0.68 mm.; bight 0.43 mm.; thickness 0.3 mm. 



Valves moderately convex, subovate, sometimes obscurely quadrate; hinge rather 

 short, straight centrally, more or less rounded at the ends; ventral margin gently 

 convex, nearly parallel with the dorsal. Horseshoe ridge comparatively small, almost 

 entirely within the post-dorsal fourth, its arms terminating near the dorsal margin 

 in two rounded elevations, the connecting loop but little elevated and in most cases 

 obscure; beneath the loop another but very faint loop-like elevation of the surface 

 may be noticed. 



The horseshoe ridge is much smaller and the bent portion much less distinct 

 than in J. crepidiformis. 



Formation and locality. Galena shales (Clitambonites bed), near Cannon Falls, Minnesota. 



Genus BOLLIA, Jones and Holl. 



Jiollia, JONES and HOLL, 1886. Ann. Mag. Nat. Hist., ser. 5, vol. xvii, p. 360 



Valves subequal, oblong or somewhat rounded, with rounded and nearly equal 

 ends and a straight hinge line; surface punctate or smooth, and bearing a large loop- 

 like or more or less horseshoe-shaped ridge; from the edges the surface rises into a 

 more or less well-developed, angular or rounded marginal ridge; the outer and inner 

 ridge often come close together ventrally, but rarely, if ever, coalesce; horseshoe 

 ridge of nearly equal strength throughout, or the ends may be bulbous and the 

 connecting bent portion relatively very thin and low. 



Type: B. uniflexa Jones and Holl. 



This genus is easily recognized by the inner or horseshoe ridge, which always 

 occupies a subcentral position with respect to the ends of the valves. The species 

 are numerous and while they may be said to adhere rather strictly to the generic 

 type, it is still true that they may be divided into three distinguishable groups. In 

 the first or typical section, the outer rim or ridge is not strong, while the inner ridge 

 has bulbous ends and is on the whole larger though the bent connecting portion is 

 narrow. In the second section, of which B. vinei Jones and Holl, may be considered 

 as typical, both the inner and outer ridges are relatively thin and small, even the 

 ends of the horseshoe ridge being but little, when at all, thicker than the rest. The 

 third section, of which B. persulcata Ulrich and B. regularis Emmons sp., are both, 

 though in somewhat different ways, representative, includes species in which the 

 inner ridge is thick without being bulbous at its ends, the marginal ridge swollen, 



