584 THE PALEONTOLOGY OF MINNESOTA. 



[Otenodonta uasuta. 



being the presence of a low triangular striated area between the beaks and the hinge 

 in the Axincea. Other Arcidce present almost equally close resemblances to C. loganL 

 Aside from the ligamental area the principal characters of Area and Isoarca are 

 practically the same as in one or another species of Ctenodonta. 



We have then three families of recent shells (as defined by Stoliczka) any one or 

 all of which, and I believe it is the latter, may have been derived from this early type. 



C. nasuta section. 

 CTENODONTA NASUTA Hall. 



PLATE XLII, FIG. 30. 



Tellinomya nasuta HALL, 1847. Pal. N. Y., vol. 1, p, 152; 1857, Tenth Eep't. Reg. Univ. N. Y., 



p. 183, ?flg. 2. (Figures 1 and 3 not strictly nasuta,) 



Ctenodonta logani SALTEB, 1851. British Asso. Rep., p. 63. (Not C. logani Salter, 1859.) 

 Isoarca logani WOODWARD. Manuel Shells, p. 269. 

 Ctenodonta nasuta SALTER, 1859. Can. Organic Remains, Dec. I, p. 35. 



Shells transversely elongate subovate', the length one-twentieth or more greater 

 than twice the greatest bight; beaks rather small, not very prominent, incurved, 

 situated about one-twelfth of the entire length in front of the middle; anterior end 

 large, broadly and regularly rounded; posterior end produced, tapering, rather nar- 

 rowly rounded at the extremity; cardinal margin nearly straight, basal line broadly 

 convex except for some distance behind the middle where it is straight or more 

 often gently sinuate. Greatest thickness near. the middle of the anterior half, 

 equalling about one-third of the length of the shell. Umbones moderately inflated, 

 the posterior cardinal slope defined by an obscure umbonal ridge, very abrupt for a 

 short distance behind the beaks, more so than on the anterior side; a broad and very 

 shallow sulcus crosses the valves obliquely from the umbones to the contraction in 

 the base. Ligament attached in a sharply defined groove on each side of the hinge 

 line, extending from the beaks about one-third of the distance to the posterior 

 extremity. Surface marked with obscure concentric lines. 



The test being thin, casts of the anterior look much like the exterior of the shell 

 itself. The muscular scars are faintly impressed and usually determined with difficulty 

 on all except the largest casts. The denticulated part of the hinge is comparatively 

 short, being but 21 mm. in length in a specimen 56 mm. long. Its upper margin is 

 nearly straight, but the lower side is rather distinctly biconvex, the plate being con- 

 stricted beneath the beaks to little more than half of its width on each side. The 

 denticles form a continuous series, are small and vertical beneath the beaks, slightly 

 oblique in front of them, and strongly curved outwards behind them. The entire 

 series, so far as observed, contains twenty-seven to twenty-nine teeth, divided almost 

 equally with respect to the beak. 



