600 THE PALEONTOLOGY OF MINNESOTA. 
(Ctenodonta simulatrix. 
CTENODONTA SIMULATRIX, #. Sp. 
PLATE XLII, FIGS. 74 and 75. 
In its general aspect this species greatly resembles C. albertina, and yet it is a 
widely distinct form, the hinge being quite different in the two forms. The hinge 
plate in C. stmulatriz is much narrower and more uniformly arcuate, the denticles 
are more numerous and the majority straight and very small. Posterior to the 
beak, beneath which the continuity of the series is slightly interrupted, there are 
about twenty-five denticles; in front of the beak the specimen preserves only six 
teeth, but, judging from other species, their number on this part of the hinge cannot 
have been less than twelve and probably was quite as many as fifteen, making a 
total for the entire hinge of from thirty-seven to forty. Comparing outlines, it will 
be found that in the present species the ends are more regularly curved and the 
beaks situated a little farther from the anterior extremity. 
Formation and locality.—Upper part of the Hudson River group near Spring Valley, Minnesota. 
C. recurva section (Palwoconcha, Miller.) 
CrenopontTa comPpREsSsA Ulvyich. 
PLATE XXXVII, FIG. 29; PLATE XLII, FIGS. 88—90. 
Tellinomya compressa ULRICH, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 216. 
Shell rather small, somewhat oblique, compressed convex, the length and hight 
respectively as twelve or thirteen is to fourteen; convexity about half the length; 
upper half triangular, the lower somewhat obliquely semielliptical; beaks small, 
compressed, acuminate, curving backward; umbones rather flat, the convex part of 
the valves terminating somewhat abruptly along the anterior and posterior cardinal 
margins. In the outline these two margins, meeting at the beaks, form an angle of 
about 85°, with the anterior gently convex and the posterior correspondingly con- 
cave, or a little straighter. Antero-dorsal edge flattened but unusually narrow, with 
- an obscure furrow on each side of the raised contact line; posterior lunette obscurely 
defined. Surface with very fine, regular, raised, concentric lines, six to eight in 1 mm. 
Hinge plate bent rectangularly, very wide in the central part; denticles mostly 
transverse to the hinge, arranged in two distinct series, increasing gradually in size 
and curvature away from the beaks, about twenty-two anterior and twelve posterior. 
A wide crescent-shaped flat space, over which the teeth do not extend, forms the 
inner border of the hinge plate. Just in front of the point of the beak, and sepa- 
rating the two series of denticles, is the narrow end of an obscurely defined, curved 
depression, extending more than two-thirds the distance across the hinge plate. 
