OSTRACODA. 673 
Dilobella typa.] 
DILOBELLA TYPA, 2. Sp. 
PLATE XLVI, FIGS. 30—34. 
S1zeE.—Length 1.0 mm.; hight 0.8 mm.; greatest thickness 0.52 mm. 
Length 0.9 mm.; hight 0.75 mm. 
Valves varying somewhat in outline, some being obscurely quadrate or subovate, 
others short-reniform; dorsal outline more or less concave at the middle and 
rounded or subangular at the ends; ventral margin strongly convex, the lower half 
of the outline being in some cases almost semicircular. A deep, subcentral, vertical 
sulcus divides the valves into two large subequal lobes. These are very prominent, 
especially at their centers, and rise abruptly from the flattened borders. At the base 
-an obscure connection between the lobes may be noticed. 
When the valves are not perfectly cleared from the matrix, some difficulty may 
be experienced in distinguishing them from the associated Ctenobolbina crassa, which 
also has a deep sulcus. But in that species the sulcus is curved and does not divide 
the lobes ventrally, and the valves are longer and differently shaped. In fact the 
two species differ so greatly that I cannot conceive how good specimens might be 
confounded. 
Formation and locality.—Upper third of the Trenton shales (Phylloporina bed), St. Paul, Minnesota. 
Genus CTENOBOLBINA, Ulrich. 
Ctenobolbina, ULRIcH, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 108. 
Carapace small, elongate-suboval, strongly convex, the posterior two-fifths more 
or less decidedly bulbous or subglobular, and separated from the remainder by a 
deep, narrow and more or less oblique sulcus extending with a gentle curve from 
the dorsal margin more than half the distance across the valves toward the postero- 
ventral border. The anterior three-fifths often with another oblique but less 
impressed sulcus. Valves equal, the dorsal margin straight, hingement simple, the 
ventral edge thick, and the true contact margins generally concealed, in a lateral 
view, by a “frill” or flattened false border; surface granulose, smooth, or punctate. 
Type: C. (Beyrichia) ciliata Emmons sp. 
Fig. 49.—a, 6, c, lateral. posterior, and ventral views of a left valve of Ctenobolbina ciliata var. 
emaciata Ulrich, x 20; shales of the Hudson River group, Savannah, Illinois. This species probably occurs 
also in the equivalent shales near Spring Valley, Minnesota. 
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