586 THE PALEONTOLOGY OF MINNESOTA. 



LCtenodontn 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. cuneiformis, 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, n. 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. 



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. 



