Report of the State Geologist. 557 



striated Longitudinally, the median ridge passing between them. 



The area is coarsely striated. 



Type, OboleUa chromcriica, Billings. 



Distribution. Cambrian (?) Lower Silurian. The descrip- 

 tion by Mickwitx of the interior of a species from the Onguliten- 

 sandstein, considered by him an Obolus (0. Quenstedti, Mickw.), 

 renders the generic distinction between Obolus and Ojbolella, for 

 the time being, somewhat obscure. Obolus QuenMedU seems to 

 possess the characters of Obolklla rather than of Obolus. The 

 determination must rest upon future studies of Obolus Apollinis, 



Eichwald. 



Elkania, Ford. 1886. 



(Plate 3, Figs. 13, 14.) 



Synonym ; Billingsia, Ford. 1886. 



Shells ovate or circular, biconvex ; shell substance largely cor- 

 neous and distinctly laminated. Apex of pedicle-valve marginal ; 

 within it is a broad subtriangular depression which merges into 

 a deep central spoon-shaped cavity. Two narrow furrows diverge 

 from its anterior extremity, separating the broad lateral muscular 

 scars. Two small submarginal grooves pass from the beak out- 

 ward, skirting the posterior portion of the lateral impressions, 

 and appear to terminate in distinct subcircular scars situated near 

 the lateral margins of the valve. 



In the brachial valve is a thickened area in the umbonal region, 

 presenting the appearance of a false cardinal area within the 

 cavity of the umbo. This area is tripartite, bearing a deep, nar- 

 row central, and two broader lateral grooves ; about its anterior 

 margin lie two broad scars of the same character as the laterals 

 of the other valve. There are also diverging furrows, which, start- 

 ing at the marginal apex, pass over the edge of the thickened area, 

 and are most deeply impressed at their anterior extremities. 



Type, Elkania desiderata, Billings (sp.). (Cambrian; one 

 species.) 



Leptobolus, Hall. 1871. 



(Plate 3, Figs. 8-12.) 

 Shells small, fragile, subovate in outline ; shell substance semi- 

 phosphatic. Exterior with concentric growth-lines. The interior 

 of the pedicle-valve shows a notably large cardinal area, which 

 is sharply grooved. Beneath this area, in the bottom of the 

 valve, is a broad depression extending nearly across the shell, and 



109 



