592 THE PALEONTOLOGY OF MINNESOTA. 



[Otenodonta nitlaa. 



end regularly rounded, or this is so only in the lower two-thirds, the curve of the 

 outline sometimes increasing in rapidity as it turns into the cardinal margin. 

 Posterior umbonal ridge prominently rounded; in front of it a very obscure mesial 

 sulcus; post-cardinal slope rather abrupt, with two obscure curved furrows, and in 

 the upper part the fulcra to which the external ligament was attached. Surface 

 marked by unequal concentric lines of growth. ' Hinge plate of moderate strength, 

 gently arcuate, slightly contracted in the middle, 15 mm. long in a specimen 25 mm. 

 in length, bearing a continuous row of teeth curving strongly inward, the whole 

 number in each valve about seventeen, of which nine are posterior; as usual, the 

 central ones are the smallest. Shell comparatively thin; muscular impressions faint. 

 The gibbosity of the shell and the unusual prominence of the umbones removes 

 this species from the C. nasuta section, while the thinness of the test and the faint 

 delineation of the muscular scars will not allow it to be placed in the C. gibherula 

 section. The natural position of the species may, however, still be considered as 

 intermediate between those two sections. 



Formation and Zocoi%.—" Upper _BufE limestone" of the Trenton formation, Beloit, Wisconsin. 

 In Canada the species occurs in the Blaclc Eiver limestone at Pauquette's Bapids, near Ottawa. 

 Mus. Reg. No. 8316—1. 



C. levata section. 

 ^ Ctendonta nitida Ulrich. 



PLATE XLII, FIGS. 44-47. 



TelUnomya nitida Ulbich, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., p. 215. 



Shell small, thin, moderately ventricose, trapezoidal or somewhat obliquely sub- 

 triangular, the antero-cardinal region somewhat alated; umbones full, beaks small; 

 closely incurved, directed slightly backward. Posterior extremity oblique, rather 

 abruptly truncated, flattened, nearly straight, pinched and projecting slightly beyond 

 the convex part of the shell in the upper half and narrowly rounded below. Ventral 

 margin gently convex, usually curving rather sharply upward at the ends. Anterior 

 end wide, rounded and most prominent in the lower half, straightened above, the 

 junction with the hinge-line subangular. Surface, excepting a few indistinct lines 

 of growth, smooth. 



Casts of the interior have strongly projecting beaks. The internal characters of 

 the shell, so far as they cah be made out from these casts, are as follows: Hinge 

 line very slightly arcuate, with eight to ten strong teeth behind the beaks and fifteen 

 or sixteen smaller ones in front of them. Anterior and posterior muscular impres- 

 sions subequal, distinct, the posterior one drawn out along the hinge margin. Above 

 the anterior pair there is another much smaller elongated pair lying close to the 

 hinge. 



