664 THE PALEONTOLOGY OP MINNESOTA. 



[Dlcranella. 



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. ? subcequata 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, n. 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 

 byrnevi Miller, he refers (op. cit., vol. 46, p. 12; 1890) to the genus sEchmina. According 

 to my estimate of these species, they should not be referred to sEchmina 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 Atchmina 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 

 Dlcranella than the other genera to which they have been referred. The answer to 



