542 THE PALEONTOLOGY OF MINNESOTA. 
(Cyrtodonta obesa. 
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 River 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. 
CYRTODONTA OBESA, ”. Sp. 
PLATE XXXIX, FIGS. 10, 11 and I2. 
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 J 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.; hight 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, 
which 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 equivalent to the Black River limestone of New York, in 
Mercer county, Kentucky. Specimens doubtfully referred to the species were found in the middle third 
of the Trenton shales at St. Paul and Preston, Minnesota. 
CYRTODONTA GIBBERA, 2. Sp. 
PLATE XXXIX, FIGS. 13—15. 
In this specimen the umbones are more inflated even than in C. obesa. They 
are also situated farther forward, the anterior end being very short and exceedingly 
obtuse. Although the posterior extremity is subangular, the form 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 
