876 THE PALEONTOLOGY OF MINNESOTA. 
[Tetranota. 
inata they continue also over the outer volution quite to the margin of the aperture. 
In T. bidorsata and T’. wisconsinensis the two central ones become more prominent 
toward the aperture, and on casts of the interior usually appear as a broad and 
more or less flat-topped single ridge. In the former the lateral ridges, though 
much less distinct, are still distinguishable in the outer half of the last whorl, but 
in the latter not a trace of them is to be seen on any part of the outer volution of 
casts of the interior, the only condition in which the species is known. The same 
is true of 7. obsoleta with the addition that the double central ridge also is much 
less developed than in the other species. 
The unusual width of the slit-band, which les between the two central ridges 
and is more or less concave—never much so,—is another important characteristic, 
though not entirely confined to Tetranota. 
Briefly the new genus differs from all the other genera of the suborder in the 
possession of four dorsal ridges. In most other respects we find that Tetranota 
agrees closely with Bucania and Megalomphala. Still there are some good additional 
differences. Thus, Bucania has a narrower slit-band and longer slit, besides stronger 
revolving surface sculpture, while Megalomphala agrees in all respects with Bucania 
excepting that it has no revolving lines. The Cambrian Owenella, though reminding 
one in a general way of T'etranota, is distinguished at once by the absence of a slit- 
band. It is scarcely necessary to extend our comparisons to such widely different 
types as Bellerophon, Bucanopsis and Protowarthia. 
As to the shells of Huphemus, about which genus we have already (see page 856) 
expressed the opinion that it is an atavistic type of Carboniferous Bellerophontide, 
they have no umbilicus and their apertures are even less expanded. Further, they 
have an inner lip slightly thickened laterally and its central portion, which bears more 
or less numerous revolving folds, is spread asa thin sheet over the inner volutions and 
part of the outer. These folds may recall the ridges of Tetranota, but are in reality 
quite different, being a feature of the ventral side of the shell in the one case and of 
the dorsal side in the other. 
Koken (loc. cit.) connects several Silurian and Devonian species of Europe with 
T. bidorsata. In this he has doubtless committed an error, while his proposal to use 
Meek’s Bucanella for the group seems to us quite unwarranted. In the first place 
his Bucanella esthona and B. subtriata Krause sp. (we are not sufficiently acquainted 
with the others to include them in our opinion) are not congeneric with 7’. bidorsata, 
being without the characteristic dorsal ridges, while they have uninterrupted straight 
revolving lines which do not occur in a Tetranota. Where the subtriata belongs is 
most difficult to say just now,* but of the esthona we may say that it is kept out of 
* We refer to this species again in our remarks on Bucanopsis. 
