586 THE PALEONTOLOGY OF MINNESOTA. 
[Ctenodonta oviformis. 
the denticles are relatively more numerous on the posterior part, there being about 
sixteen or seventeen on this side of the beaks to about ten in front of them. Casts 
of C. nasuta again exhibit a rather well marked lanceolate depressed area extending 
posteriorly from the beaks about half way to the extremity of the cast. In C. sub- 
nasuta the corresponding area is not lanceolate, but consists of a furrow on each 
side of the raised hinge line running backwards almost to the extremity. The fol- 
lowing two species also are rather closely related, but are readily enough distin- 
guished by their shorter form and lesser convexity. 
Since the above was written, I have found among my unworked material from 
the middle third of the Trenton shales in Goodhue county, two valves that may 
represent an earlier form of this species. Artificial casts of the interior of these 
valves closely resemble the Galena shales type of the species, the only difference 
being that the central part of the casts is not quite so full and the basal line less 
straightened in the posterior half. There is also a flattened rim along the ventral 
border that is not seen in the type. In these features the valves remind somewhat 
of C. oviformis, but they cannot belong to that species, since they are too narrow 
posteriorly and have the beaks situated more anterior to the center. The hinge is 
rather well preserved on both the valves, each having about twenty-six denticles, 
nine of them in front of the beaks. Of the latter the anterior five are larger than 
any of the others. The hinge, on the whole, resembles that of C. cunetformis, but 
the anterior teeth are larger and the beaks situated farther forward. 
Formation and locality—The type is from the Galena shales near Cannon Falls, Minnesota. 
CTENODONTA OVIFORMIS, 2. Sp. 
PLATE XLII, FIG. 29. 
Shell small, compressed convex, transversely ovate, the ends rather narrowly 
_ rounded, subequal, the anterior a trifle wider and shorter than the posterior, the 
base almost regularly convex, the hinge line gently arcuate, and the beaks rather 
small, scarcely prominent and situated slightly in front of the midlength. Muscular 
scars comparatively distinct. Number of teeth and surface unknown, but the cast 
is marked with several obscure concentric furrows. Length, 9.2 mm.; hight about 
6 mm.; thickness, 3.8 mm. P 
This small shell is relatively shorter, less produced and wider posteriorly, and 
more rounded in the basal outline than C. nasuta and C. subnasuta. Collectors will, 
I think, find little trouble in recognizing it. 
Formation and locality.—Galena shales, near Cannon Falls, Minnesota. 
