672 THE PALEONTOLOGY OF MINNESOTA. 



[Drepanella bigenerls. 



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, n. sp. 



PLATE XLIV, FIGS. 20-22. 



SIZE. Length 2.3 mm.; night 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, 

 l-'.ntnmidella 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. 



