GASTROPODA. 875 
Tetranota.| 
to side,—at any rate they are much less sinuate in the middle than is the margin of 
the aperture. 
The ventricose volutions, shallow sinus and broad transverse furrows (on casts) 
are the distinctive features. Very young specimens may look much like the largest 
of P. pervoluta, but the adult form, which is all we have seen, is certainly quite 
distinct. P. obesa should be compared with the British P. bilobata Sowerby sp. (see 
fig. 3, p. 869) because it is, according to our opinion, the nearest known American 
representative of that much (mis-) quoted species. And still the American form 
is distinct if only for the reason that the apertural sinus is deeper in the dilobata. 
Formation and locality.—Lower beds of the Trenton group, Burgin and Danville, Kentucky. 
Collection.—K., O. Ulrich. 
Genus TETRANOTA, n. gen. 
Bucania (part.), HALL, 1847, Pal. New York, vol.i, p. 186. 
Bellerophon (part.), WHITFIELD, 1874, Geol. Wis., vol. iv, p. 223. 
Bucanella (part.), KokEN, 1889, Neues Jahrbuch f. Mineralogie, etc., Beilageband vi, p. 389. 
For generic characters see page 849. : 
Superficially examined the systematic position of this genus will probably 
appear to be between Megalomphala and Huphemus. The relation to the latter how- 
ever is more apparent than real. Comparing the five species now referred to 
Tetranota we find that the umbilicus, though always present, is yet somewhat 
variable, being very large in T. macra, T. sexcarinata and T. bidorsata, moderate in 
T. obsoleta, and comparatively small in 7. wisconsinensis. The verticai expansion of 
the aperture is extremely limited in all save the last mentioned, and in this the 
vertical diameter of the volutions also is greatest. The external sculpture, aside 
from the revolving ridges, consists chiefly of lines of growth. These are always 
regular and sharp, but vary in strength with the species. 
While strong revolving lines like those of Bucania or of Bucanopsis do not occur, 
it is nevertheless a fact that in all cases of Tetranota preserving the external layer 
of the shell, or an impression of the same, we observed a row of minute prominences 
on each of the sublamelliform lines of growth. Ina few instances we found further 
that these prominences were in reality the anterior terminations of very fine short 
striz crossing from line to line. This style of sculpture, excepting that it is much 
finer, is essentially the same as that marking the B. lindsleyi section of Bucania. 
We attach great weight to this point and believe that it proves our association of 
the genus with the Bucaniide to be well founded. 
The four revolving dorsal ridges, which we regard as the principal peculiarity 
of the genus, are always distinctly developed on the inner volutions. In T. sexcar- 
