LAMELLIBRANCHIATA. 237 
Oyrtodonta janesvillensis.] 
the anterior basal margin. Surface nearly smooth in the young and middle stages 
but with age one or more very strong marginal imbrications are developed. In 
aged examples the anterior end is proportionally narrower than in younger ones. 
Hinge plate of moderate length with a narrow ligamental area. Cardinal teeth 
three in each valve, sub-horizontal, their inner ends thickened and curved down- 
ward. Posterior teeth two in the left and three in the right valve. Both muscular 
impressions faint. Shell rather thin. 
All the Kentucky types of this species retain the shell and in the absence of 
unquestionable casts of the interior for comparison with the Minnesota specimens 
provisionally referred here, there may well be some doubt regarding the actual ex- 
istence of the species within the borders of the state. The cast represented by fig. 
30 exhibits certain peculiarities that it seems scarcely likely would occur jn casts of 
the Kentucky form. Thus the outline is less concave in front of the umbones and 
the length of the shell less than it ought to be ina specimen of this size. The 
original of figure 45, which is from the Trenton limestone at Cannon Falls, also 
differs a little, but in this case oblique pressure has produced distortion that may 
account for the differences. 
This species is closely related to both C. huronensis and canadensis which Billings 
described from the lower Trenton or Black River limestone of Lake Huron. Com- 
pared with authentic specimens the first proves to be narrower posteriorly and the 
second wider in front. In the latter the umbones are also more inflated. The 
hinges of the two species as figured by Billings are also somewhat different. 
Formation and locality.—The types of the species were found in the Birdseye and lower Trenton 
limestone near High Bridge, Kentutky. The original of Figure 30 is from the middle third of the Trenton 
shales at St. Anthony Park, St. Paul. That of Figure 45 from the Trenton limestone at Cannon Falls. 
CyRTODONTA JANESVILLENSIS, 22. Sp. 
PLATE XXX1X, FIGS. 26 and 27. 
Comp. Cyrtodonta huronensis Billings, 1858, Can. Nat. and Geol., vol. iii, p. 432. 
Shell of medium size, strongly convex, somewhat obliquely ovate, widest 
posteriorly, the hight and length about as two is to three. Outline almost 
uniformly rounded for an oval, with a slight prominence at the beaks and occasion- 
ally at the posterior end of the hinge line. Anterior end very short. Beaks a little 
compressed, rather small, incurved, projecting but little above the hinge. In casts 
of the interior the umbonal ridge is:strongly and the surface in front of it slightly 
depressed. Anterior adductor scar, well defined, ovate, small, not more than half 
the size of the posterior scar. The latter as usual is scarcely distinguishable. 
Pallial line well marked, particularly in the basal and anterior parts. Hinge plate 
