CRUSTACEA— OSTRACODA. 353 



XXVII. Tetradella Ulrich. 

 Valves marked by four more or less curved vertical ridges ven- 

 trally united; one or both of the inner ridges sometimes duplex 

 (7". lunatifera) or all four may be split up into separate nodes 

 {T. dissecta Kr.). Eighteen species. Ordovicic-Siluric. 



59. T. quadrilirata (H. and W.)- (Fig. 1658,/,^.) Ordovicic. 

 Posterior and ventral ridges with blunt spinose processes; the 



second of the ridges from front dividing at base. (Type of genus.) 

 Black River of Minnesota; Stones River and Richmond of Cin- 

 cinnati region. 



60. T. lunatifera Ulrich. (Fig. 1658, /i.e.) Ordovicic. 

 Without spinous processes; second and fourth ridge duplex. 

 Trenton of Minnesota; Richmond of Cincinnati region. 



XXVIII. Ctenobolbina Ulrich. 

 Shell oblong or subovate ; posterior two fifths generally bulbous 

 or subglobular, separated from the remainder by a deep, obliquely 

 curved sulcus extending from the dorsal margin more than half 

 across the valves towards the postventral border; anterior three- 

 fifths often with another oblique but less impressed sulcus; dorsal 

 margin long and straight, hingement simple; ventral edge thick, 

 the true contact margins generally concealed in a lateral view by a 

 frill or flattened false border. Fifteen species. Ordovicic- 

 Carbonic. 



61. C. fulcrata Ulrich. (Fig. 1660, r,r',.y.) Ordovicic. 

 Anterior sulcus ill-defined; posterior border thick with coarse 



pits. 



Black River of Minnesota. 



62. C. ciliata (Emmons). (Fig. 1660, f, ^'.) Ordovicic. 

 Larger than preceding ; posterior sulcus crescent-shaped, narrow 



and deep, anterior one shallower and narrower. (Type of genus.) 

 Eden of Cincinnati region. 



6^. C. granosa Ulrich. (Fig. 1658, u.) Devonic. 



With pronounced, slightly lobate, frill extending ventrally and 

 half way to posterior dorsal margin. 



New Scotland of New York. 



^3 



