596 THE PALEONTOLOGY OF MINNESOTA. 



[Otenodonta calvlnl. 



hinge, with its oblique teeth, is very different. In C. socialis the outline is more 

 regularly oval and the beaks situated as much behind the center as they are in front 

 of that point in C. fecunda. In C. simulatrix the anterior end is wider, the muscular 

 scars deeper and the hinge more numerously denticulated. Similar differences dis- 

 tinguish C. albertina, a well marked species having also a stronger hinge and genicu- 

 lated teeth. The following species, C. calvini, is probably nearer than any other 

 species now known. 



Formation and locality. Very abundant in the lower so-called Maquoketa shales near Dubuque, 

 Iowa; Scale's Mound, Illinois, and Platte's Mound in Lafayette county, Wisconsin. The species occurs 

 also in Fillmore county, Minnesota, in equivalent beds (Hudson River group), though but rarely. 



CTENODONTA CALVINI, n. sp. 



PLATE XLII, FIGS. 61-64. 



Shell subquadrate-ovate, about 15 mm. long, 12 mm. high and 6 mm. thick; 

 anterior end rounded, a trifle narrower than the slightly truncate posterior end; 

 the latter is a little oblique, gently convex except below, where the outline turns 

 rather sharply into the broadly rounded base; above it forms an obtusely angular or 

 rounded junction with the straight post-cardinal margin; in front of the scarcely 

 prominent beaks, which are situated about one-third of the entire length behind the 

 anterior extremity, the outline is more or less concave. Surface of valves rather 

 uniformly convex, with the posterior umbonal ridge strongly rounded, though in no 

 case conspicuous. External surface markings not observed. Casts of the interior 

 exhibit a few concentric undulations, and in the central and ventral parts a variable 

 number of obscure rays. The test seems to have been unusually thin. Hinge plate 

 very narrow, bent at the beak, straight behind, gently concave in front; denticles 

 small, oblique, about sixteen posterior and eight or ten anterior in each valve. 

 Adductor muscular scars very slightly impressed, the posterior one extended above, 

 larger and longer than the anterior, and placed in the middle of the cardinal slope, 

 so that its long axis is parallel with the umbonal ridge; several small umbonal scars 

 may be observed. 



This fine species, though closely related, is at once distinguished from C. fecunda 

 by its greater posterior hight and larger size. The convexity of the valves also is 

 somewhat less, and other differences may be detected in comparing the figures 

 of the two species on plate XLII. A nearer form, perhaps it ought to be called a 

 variety, occurs in the lower beds (Utica horizon) of the Cincinnati group at Coving- 

 ton, Kentucky. This has exceedingly fine and crowded concentric strife, crossed by 

 more distant radiating lines. Casts of the interior have about the same shape as 

 C. calvini, but they are all much smaller, the largest having a length of only 7 mm. 



