592 THE PALEONTOLOGY OF MINNESOTA. 
[Ctenodonta nitida. 
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. gibberula 
section. The natural position of the species may, however, still be considered as 
intermediate between those two sections. 
Formation and locality.—‘‘ Upper Buff limestone” of the Trenton formation, Beloit, Wisconsin. 
In Canada the species occurs in the Black River limestone at Pauquette’s Rapids, near Ottawa. 
Mus. Reg. No. 8316—1. 
C. levata section. 
Crexpowra nitipa Ulrich. 
PLATE XLII, FIGS. 44—47. 
Tellinomya nitida ULRICH, 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 ineurved, 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 can 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. 
