542 THE PALEONTOLOGY OF MINNESOTA. 



, . [Oyrtodonta o1)esa. 



to the genus Vanuxemia agree even more closely in their outlines, but in all of them 

 the shell is much thicker and the hinge generically different. Of all known species 

 the two next described are to be considered as the nearest. 



Formation and locality.— The real types of the species were obtained from Mercer county, Ken- 

 tucky, where they were found in a cherty bed equivalent to the Black Eiver limestone of New York. 

 Two specimens, both a little larger than the Kentucky types, were collected in Minnesota. Both are 

 from the middle third of the Trenton shales, one at Minneapolis, the other near Fountain. 



Mus. Reg. No. 8336. 



Ctrtodonta obesa, n. sp. 



PLATE XXXIX, FIGS. 10, 11 and 12. 



This species is, so far as our knowledge extends, very closely allied to C. rotulata. 

 It is also associated with it in both Kentucky and Minnesota, but I cannot say that I 

 experienced much trouble in separating them. C. obesa is more gibbous and oblique, 

 the anterior end is shorter and much more obtuse in a cardinal view, the posterior 

 cardinal slope narrower and scarcely to be described as alate, while the outline at 

 this extremity of the hinge is more rounded; the entire outline is to be called 

 broadly ovate rather than subcircular. The umbones also are more prominent and 

 inflated. 



Length, 14 mm.; from beak to posterior extremity, 14 mm.; bight at center of 

 shell, 11.; thickness, 10.5 mm. In another specimen these dimensions are respectively 

 14.2, 14, 11 and 10 mm. 



The above measurements are furnished by two silicified examples from Kentucky, 

 wbich are to be regarded as the types of the species. Besides these two evidently 

 young shells from Minnesota localities are referred here provisionally. They are 

 too oblique for C. rotulata and have not the proper shape for C. cingulata. The out- 

 line is very nearly as in C. obesa, but they differ from the Kentucky specimens in 

 being less gibbous, especially in the umbonal region. 



Formation and locality.— In cherty beds equiyalent. to the Black Eiver limestone of New York, in 

 Mercer county, Kentucky. Specimens doubtfully referred to the species were found in tlie middle third 

 of the Trenton shales at St. Paul and Preston, Minnesota, 



Otetodonta gibbera, n. sp. 



PLATE XXXIX, PIGS. 13-15. 



In this specimen the umbones are more inflated even than in G. obesa. They 

 are also situated farther forward, the anterior end being very short and exceedingly 

 obtuse. Although the posterior extremity is subangular, the fq,rm on the whole is 

 more rotund, the hight of the shell being greater. C. rotulata is much less gibbous 

 in the umbonal and central regions, less oblique and a little longer, particularly in 



