668 THE PALEONTOLOGY OF MINNESOTA. 
[Jonesella obscura. 
JONESELLA OBSCURA, 2. Sp. 
PLATE XLIV, FIGS. 17-19. 
SizeE.—Length 0.68 mm.; hight 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. 
Bollia, 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.unifleca 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, 
