GASTROPODA. 877 
Tetranota bidorsata.] 
Bucania by its wide slit-band and straight revolving lines, while the first character, 
together with its different (Bucania-like) aperture excludes it from Bucanopsis. 
Having to describe a similar species from Minnesota, we have concluded to establish 
a new genus, Kokenia, for their especial benefit. 
The vertical range of three of the species of Tetvanota is more extended than 
usual with Bellerophontacea. T. obsoleta occurs in the Vanuxemia bed of the Stones 
River group, in the Ctenodonta bed of the Black River group, and reappears; appar- 
ently very slightly modified, in the Utica group at Cincinnati; 7. bidorsata occurs in 
the lowest division of the Stones River group of Tennessee, in the Black River group 
in Minnesota and Canada, and in the Trenton of Minnesota, Kentucky, Tennessee, 
New York and Canada; 7’. sevcarinata is found in the Vanuxemia bed in Minnesota, 
Wisconsin and Illinois, in the “Glade limestone” of Tennessee, and in the Fusispira 
bed of the Trenton in Minnesota. T'. wisconsinensis is as yet known only from the 
Vanuxemia bed in Minnesota, Wisconsin and Illinois, while 7’. macra occurs in the 
same bed at Minneapolis. 
TETRANOTA BIDORSATA Hall. 
PLATE LXV, FIGS. 10—18. 
Bucania bidorsata HALL, 1847, Pal. N. Y., vol. i, p. 186. 
Shell usually about 12 mm. in hight, but the hight may exceed 20 mm., and 
occasionally reaches 25 mm.; volutions two and a half to three and a half, vertically 
compressed, sublunate in section, the width for the inner volutions or in young 
specimens a little greater than twice the hight; in old examples the increasing 
altitude of the centro-dorsal ridges causes the width just behind the aperture to be 
proportionally somewhat less; umbilicus large, deep, rather sharply defined, the 
width generally about half of the greatest diameter of the shell; the latter dimension 
is to the greatest width of the aperture about as three is to four. Aperture some- 
what abruptly expanded laterally, the hight and width about as three is to seven; 
slightly indented by the preceding whorl; lips thin, the outer one with a moderately 
deep emargination, taking up between one-fourth and one-third of the anterior 
outline; depth of the same about one-fifth less than its width. Dorsum with four 
strong revolving ridges, the two central ones nearer each other than to the lateral 
ones, and higher, the altitude also increasing gradually to the aperture; between 
them lies the broad slit-band which is more distinctly concave on the shell than on 
internal casts, the double ridge in the latter, particularly near the aperture, often 
appearing as a broad and more or less flat-topped single ridge; on each side of the 
central ridges there is first a broad groove, then an obtusely angular ridge, and 
