876 THE PALEONTOLOGY OF MINNESOTA. 



[Tetranota. 



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 ki which the species is known. The same 

 is true of T. 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 lies 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 Tetranota, 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 Euphemus, about which genus we have already (see page 856) 

 expressed the opinion that it is an atavistic type of Carboniferous Bellerophontidce, 

 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 as a 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 

 hi.s Bucanella esthona and B. stibtriata Krause sp. (we are not sufficiently acquainted 

 with the others to include them in our opinion) are not congeneric with T. 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 BucanojMl*. 



