600 THE PALEONTOLOGY OP MINNESOTA. 



ICtenodonta slmulatrix. 



CTENODONTA SIMULATRIX, n. 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. simulairix 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 (Palceoconcha, Miller.) 

 CTENODONTA COMPRESSA Ulrich, 



PLATE XXXVII. FIG. 29; PLATE XLII, FIGS. 8890. 



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 bight 

 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. 



