540 THE PALEONTOLOGY OF MINNESOTA. 



ICyrtodonta affinls. 



different cardinal teeth. In all these respects the shell agrees much better with 

 an authentic example of C. carinata Billings, now before me, and as both the 

 Palwarca ventricosa of Hall and the Cyrtodonta subcarinala of Billings are from the 

 Trenton limestone in the northern part of Lake Huron, they are probably indentical. 



Formation and locality. Lower Trenton limestone Dunleith, Illinois; Beloit, Janesville and Mineral 

 Point, Wisconsin; Cannon Falls and Minneapolis, Minnesota. 



CYRTODONTA OBLIQUA Meek and Worthen. 



PLATE XXXIX, FIG3. 35 and 36. 



Cypricardites obliquus MEEK and WOKTHEN, 1868, Geol. Sur. 111., vol. lii, p. 311. 



Of this species I have seen only the original type figured and described by 

 the authors. Their figures being unsatisfactory, it seemed worth the while to 

 prepare others, especially as the species may at any time be found within the 

 limits of the State. It is to be looked for in the middle and lower beds of the 

 Galena in Fillmore county. The type specimen is from the Galena at Scales 

 Mound, Illinois, and is now preserved in the Illinois State Museum. 



CYRTODONTA AFFINIS, n. sp. 



PLATE XXXIX, FIGS. 20-23. 



Shell small, rather compressed convex, obliquely subovate, alated and much 

 the highest posteriorly. Dorsal margin straight or very gently arcuate, rather long, 

 not passing gradually into the broadly and uniformly rounded posterior margin, the 

 junction being obtusely angular; ventral margin but little convex, ascending rapidly 

 to the small and narrowly rounded anterior end. Beaks small, projecting very little; 

 umbones compressed, due to a flattening of the antero- ventral slope; umbonal ridge 

 moderately distinct in the upper half; cardinal slope gently concave; greatest 

 thickness on the umbonal ridge above and a trifle in front of the center of the 

 valves. Surface with fine indistinct concentric strh and distinct sublamellose lines 

 of growth. Hinge of moderate thickness; cardinal teeth small, short, four in each 

 valve; posterior lateral teeth very slender, four in the right valve. Muscular im- 

 pressions rather faint, not well determined. Length 20 mm.; posterior (greatest) 

 hight 15 mm.; anterior highest 10 mm.; entire thickness 6.5 mm. 



A variety reappears in the middle Galena. This is relatively more convex and 

 not quite as high posteriorly. Length 15 mm.; hight 10 mm.; thickness 7 mm. It 

 may be distinguished as var. fillmorensis. 



The typical form of this species is associated and was at first confounded with 

 Mutltcrlii ruyosa. Aside from the hinge, which is of course very different in the two 

 forms, the Mather in is distinguished by its much shorter, subtruncate anterior end. 



