Uilobella typa.] 



OSTRACODA. 



673 



DiLOBELLA TYPA, W. Sp. 



PLATE XLVI, FIGS. 30-34. 



Size. — 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 dilBculty may 

 be experienced in distinguishing them from the associated Gtenoholhina 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.— Vitper third of the Trenton shales (Phylloporina hed), St. Paul, Minnesota. 



Genus CTENOBOLBINA, Ulrich. 



Ctenobolbina, Uleich, 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: G. (Beyrichia) ciliata Emmons sp. 



:^.rt»»t^' i,i, ii fc'g f rt*» t;»2:i 



Fig. 49.— a, b, c, lateral, posterior, and ventral views ol a left valve of Ctenobolbina ciliata var. 

 emaciata Ulrich, x 26; shales of the Hudson River group, Savannah, Illinois. This species probably occurs 

 also in the equivalent shales near Spring Valley, Minnesota. 

 43- 



