OSTRACODA. 675 
Ctenobolbina crassa. | 
of lobes terminating in a thin, flat or raised, border, supported in the hinder part by 
five, equidistant ribs or walls, thus forming as many small cavities in the posterior 
edge of each valve; surface smooth. 
The small cavities in the posterior half of the edge remind of Tetradella quadri- 
lirata, but here the resemblance ceases for they are widely different in all other 
respects. These cavities and the relative narrowness of the posterior bulb, together 
with other peculiarities, distinguish C. fulcrata from C. duryi Miller sp., a Cincin- 
nati species that resembles fig. 8 more closely than does any other one of the genus. 
When however it comes to actual relationship, the next to be described is doubtless 
the nearest. 
Formation and locality—Upper third of the Trenton shales (Phylloporina beds), St. Paul and Can- 
non Falls, Minnesota. 
CTrENOBOLBINA CRASSA Ulrich. 
PLATE XLIV, FIGS. 12—16. 
Jonesella crassa ULRICH, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 128. 
S1zE.—Length 0.94 mm.; hight 0.60 mm ; thickness 0.60 mm. 
Length 0.80 mm.; hight 0.52 mm.; thickness 0.46 mm. 
This species is closely related to C. fulcrata, and when the edges are obscured by 
the matrix, it is difficult to distinguish from one of the varieties of that species. 
But when the posterior edge is visible the difficulties vanish, there being no supports 
nor cavities in the thick edge of C. crassa (compare figs. 13 and 16 with fig. 9, pl. 44.) 
Among other differences I may mention that in C. crassa the valves are constantly a 
little smaller, the sulcus wider, and the lobes more prominent, especially at the 
ventral edge. The lobes are also more compact and ridge-like, producing an effect 
that reminds so much of the “horseshoe” ridge of Jonesella, that I at first regarded 
the species as belonging to that genus. But that was before I knew of its close 
relationship with C. fulcrata. 
Formation and locality.—Associated with the preceding in the upper third of the Trenton shales at 
St. Paul and Cannon Falls, Minnesota. 
Genus CERATOPSIS, n. gen. 
Tetradella (part.) ULRicH, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 112. 
Beyrichia (part.) BARRANDE, HALL and WHITFELD, MILLER, JONES, and other authors. 
Valves somewhat obliquely subovate, widest posteriorly, straight dorsally, with » 
a thick rounded semicircular marginal ridge, and two submedium ridges extending 
obliquely upward from the marginal ridge, the anterior one reaching the dorsal edge, 
the other shorter and smaller; post-dorsal end of marginal ridge raised into a strong 
