LAMELLIBKANCHIATA. 237 



Oyrtodonta janesvUlensis.l 



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 in 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 in a 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 looality.—Tbe 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 Pari?, St. Paul. That of Figure 45 from the Trenton limestone at Cannon Falls. 



Cyetodonta janbsvillensis, n. sp. 



PLATE XXXIX, FIGS. 26 and 27. 



Comp. Cyriodonta 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 



