604 THE PALEONTOLOGY OF MINNESOTA. 



[Ctenodonta similis. 



CTENODONTA SIMILIS Ulrich. 



PLATE XLII, FIGS. 102-106. 







Tellinomya similis ULRICH, March 3, 1892. Nineteenth Ann. Rep. Geol. and Nat. Hist. Sur. Minn., 



p. 220. 

 Tellinomya (Nucula) lepida SAKDESON, April 9, 1892. Bull. Minn. Acad. Nat. Sci., vol. iii, p. 339. 



Shell small to medium size, moderately ventricose, subtriangular, the length 

 and hight respectively as five or five and a half is to six. Umbones full, rounded, 

 the rostral portion rather strongly recurved, with the beaks small and projecting 

 slightly above the hinge. Antero-dorsal edge convex, thick, flattened, but not 

 sharply defined. Postero-dorsal edge rather strongly concave, impressed so as to 

 form an illy-defined imperfect lunette. Anterior outline almost uniformly convex, 

 curving neatly into the well rounded ventral margin; posterior side rather narrowly 

 rounded. Surface of valves almost uniformly convex, highest a little above the 

 center, generally with a few well marked varices of growth and with finer concentric 

 lines in the lower part. Hinge plate of moderate strength, with numerous small 

 teeth (thity-five to forty -two); in the largest examples seen with about twenty-seven 

 anterior and fifteen posterior to the beak; posterior teeth the largest. Muscular 

 scars moderately impressed, always distinguishable. 



The shape of this shell is exceedingly like that of C. astartiformis Salter, though 

 as a rule proportionally a little longer and scarcely so ventricose. The posterior 

 lunette also is somewhat deeper, but the principal differences lie in the hinge. The 

 hinge plate, namely, in Salter's species, is somewhat stronger, while the denticles 

 are more bent, larger and less numerous. The teeth, furthermore, are largest on the 

 anterior side, while in C. similis the opposite is the case. It is also very much like 

 the associated C. recurva, but is distinguished by being a little higher, more uniformly 

 rounded on the anterior side and without the anterior sulcus. More important dif- 

 ferences are the greater tumidity of the umbones, less prominent beaks, less strongly 

 defined anterior and posterior lunettes and weaker hinge plate. Oasts of the interior 

 are separated chiefly by the greater thickness of the rostral portion. They are also 

 nearly always of smaller size than those of C. recurva. 



Formation and locality. Upper beds of the Hudson River group, Spring Valley and other parts 

 in Fillmore county, Minnesota, and at Blanchester, Ohio. 



CTENODONTA OBLIQUA Hall. 



PLATE XLII. KIGS. 83-87, 



. 



Nucula obliqua HALL, 184r>. AIIKT. .lour. Sci. and Arts. vol. xliii, p. 292. 



Tellinomya f obliqua MEKK, 187.'). Pal. Ohio, vol. i, p. 139. 



Palaeoconcha obliqua and P. fabcri MILLEII, 1889. North Amer. Geol. and Pal., p. 498. 



Shell very small, broadly acuminate-subovate; or, without the triangular rostrum, 



