590 THE PALEONTOLOGY OF MINNESOTA. 



[Ctenodonta lon.tfii. 



anterior extremity. Posterior end long, subtriangular in outline, with the extremity 

 subacute and the dorsal side almost straight (faintly convex) from the beaks backward ; 

 ventral margin broadly rounded, semielliptical; anterior margin nicely rounded. 

 Post-cardinal side thick, with a large, sharply defined and slightly concave area 

 reaching from the beaks to near the posterior extremity of the shell. Surface gently 

 convex, scarcely sloping toward the postero-cardinal area, marked with exceedingly 

 fine concentric striae and a few stronger lines of growth. Hinge plate strong, 

 abruptly bent, the posterior part nearly twice as long as the anterior, the two 

 parts forming an angle of about 105; denticles very little curved, in two distinct 

 series, those in each row set transversely on the hinge plate, about eleven in the 

 anterior series and nineteen or twenty in the posterior series. Muscular impressions 

 very deep, bordered on their inner sides by strong ridges and set into the wide exca- 

 vated ends of the hinge plate; pallial line simple, faint, submarginal. 



The interior of this strongly marked species proves to be quite different from 

 what I expected when I described it from the exterior alone. Instead of showing 

 relations to C. levata, excellent valves since obtained prove the species to be nearer 

 C. gibberula and C. contracta. Still, C. planodorsata occupies a somewhat isolated posi- 

 tion. In the first place, casts of the interior, with their very prominent muscular scars, 

 remind at once of the present section of the genus. The hinge, however, is very 

 different from that of the typical members of the section, the plate being widest 

 under the beaks instead of much the narrowest, and the series of denticles very 

 abruptly divided into two sets, the whole hinge, therefore, being much more as in 

 the C. recurva section. C. longa has similar characters, as appears to be the case 

 also with an undescribed species from the Trenton of Tennessee, so' that it might 

 have been well to institute another section of the genus. But as these sections are 

 merely temporary natural groupings of the species, pending a more thorough study 

 of the whole family, the ommission cannot be of much consequence. 



The species is so easily recognized by the flat dorsum that comparisons are quite 

 unnecessary. 



Formation and locality. In the upper part of the middle third of the Trenton shales at severa 

 localities in Goodhue county, Minnesota. 



CTENODONTA LONQA Ulrich. 



PLATE XXXVII, FIGS. 30 arid 31. 



Tellinomya tonga ULRICH, 1892. Atner. Geol., vol. x, p. 103. 



Shell small, compressed, elongate-elliptical, the length equalling a little more 

 than twice the greatest bight. Beaks small, situated about one-fourth of the entire 

 length from the anterior extremity. Cardinal line, on the whole, very slightly 



