672 THE PALEONTOEBOGY OF MINNESOTA. 
(Drepanella bigeneris. 
The form, prominence and bilaterally symmetrical disposition of the nodes and 
ridge give this species a very distinct and striking appearance, and among all the 
numerous Silurian Ostracoda not one is known with which it might be confused. 
Formation and locality.—Upper third of the Trenton shales (Phylloporina bed), St. Paul and near 
Cannon Falls, Minnesota. 
DREPANELLA BIGENERIS, ”%. Sp. 
PLATE XLIV, FIGS. 20—22. 
Size.—Length 2.3 mm.; hight 1.36 mm.; greatest thickness 0.95 mm.; average thickness, not 
including nodes and: ridges, about 0.5 mm.; thickness of posterior and ventral edges 
about 0.6 mm. 
Valves oblong-subquadrate, longest in the lower half, the ends nearly equal and 
converging slightly in the upper half; back straight, the posterior extremity suban- 
gular, the anterior rounded; ventral outline very gently convex; marginal or “sickle- 
shaped” ridge sharply defined, extending farther up on the anterior side than in any 
of the other species of the genus. Two thick nodes or lobes, the anterior one the 
longer and more prominent, are connected below by a narrow loop-like thickening 
of the lower border of the median depression or sulcus, the whole producing precisely 
the effect of the “horseshoe” ridge of Bollia. 
My reasons for placing this fine species under Drepanella instead of Bollia 
are given in the remarks following the generic description. The specific characters 
are well marked and conspicuous, so that there is little difficulty in distinguishing 
the species from the rest of the Minnesota Ostracoda. 
Formation and locality.—Lower limestone of the Trenton formation, Minneapolis and St. Paul, 
Minnesota. 
Genus DILOBELLA, n. gen. 
Carapace small, equivalved, subovate or somewhat reniform in outline, the back 
straight or faintly concave; valves bilobed, the lobes subequal, very large, and almost 
completely separated by a deep subcentral vertical sulcus; edges thin, simple; 
surface smooth. 
Type: D. typa, n. sp. 
I find myself obliged to erect a new genus for this remarkable ostracode. A 
slight resemblance to certain forms of Bollia may be noticed, but the lobes are 
altogether too large for that genus. That it cannot belong to either Entomis, 
Entomidella nor Ctenobolbina, the only other genera with which it might be compared 
must be evident to anyone who has paid attention to this class of fossils. As to its 
affinities, they are obscure. Because of the slight basal connection between the lobes, 
it may be regarded provisionally as an extravagant development of the Bollia type 
of structure. . ; 
