Ctenobolbina cra.sa.J OSTKACODA. 675 



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 ceas-es 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 ZocaJ%.— Upper third of the Trenton shales (Phylloporina beds), St. Paul and Can- 

 non Tails, Minnesota. 



Ctenobolbina crassa TJlrich. 



PLATE XLIV, FIGS. 12—16. 



Jonesella crassa Ulkich, 1890. Jour. Gin. Soc. Nat. Hist., vol. xiii, p. 123. 



Size.— 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, pi. 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 Zocah'f^.— 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.) Ulkich, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 112. 



Beyriohia (part.) Baebandb, 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 



