664 THE PALEONTOLOGY OF MINNESOTA. 
[Dicranella. 
area convex, about 0.3 mm. wide along the ventral edge, the width decreasing rapidly 
in nearing the dorsal angles; ventral two-thirds surmounted by a narrow, crescent- 
shaped thickening, depressed centrally, and marked with rather large elongated and 
concentrically arranged pits. 
The affinities of this remarkable species are very uncertain, and it is only 
provisionally placed under Eurychilina. Perhaps it can go into the new genus with 
E. ? subequata and the other species mentioned on p. 659. On the other hand, the 
two dorsal tubercles may indicate a remote relationship with Ulrichia. Whatever 
position it may ultimately occupy in classification, it is safe to say that it now 
stands quite alone. 
Formation and locality.—Upper third of the Trenton shales (Phylloporina bed), St. Paul and near 
Cannon Falls, Minnesota. 
Genus DICRANELLA, n. gen. 
Valves equal, similar to those of Primitia, excepting that they have “frilled” 
margins, while each side of the sulcus is raised into a more or less prominent horn-like 
process. These prominences are directed dorsally and may be subequal, or the 
posterior one may be much the smaller. 
Type: D. bicornis, nu. sp. 
Though doubtlessly embracing a good generic type, it is as yet scarcely possible 
to give a satisfactory diagnosis of this new genus. Two of the following species, the 
type and D. spinosa, are certainly congeneric, and the third, D. marginata, probably 
also. But the fourth, D.? simplex, is one of four species which, while closely related 
among themselves, are, to say the least, only doubtful members of this genus. Two 
of these four species Prof. T. Rupert Jones recently described as’ Ulrichia nicholsoni 
and U. marrii (Quart. Jour. Geol. Soc., vol. 49, p. 294; 1893) while the third, Leperditia 
byrnesi Miller, he refers (op. cit., vol. 46, p. 12; 1890) to the genus chmina. According 
to my estimate of these species, they should not be referred to Achmina because, 
instead of a single horn-like prominence rising from the center of the dorsal slope, 
they have two, one subcentral, the other behind it, while between them there is 
more or less of a notch or sulcus. In Ulrichia the two generic knobs are merely 
rounded prominences or tubercles on the surface of the valves, never horn-like, nor 
are their apices turned toward or beyond the dorsal margin. The probabilities are 
that the affinities of Achmina and Ulrichia are widely different, and it would be good 
policy, for the present at least, to restrict their application to forms in which the 
generic features are sharply defined. : 
As to these four doubtful species, they are, it seems to me, clearly nearer 
Dicranella than the other genera to which they have been referred. The answer to 
