Report of the State Geologist. 553 



dividing wall being continued as a short septum in front of the 

 platform. Muscular scars of the platform usually extremely 

 faint, but under favorable preservation divisible into median and 

 lateral components. Pallia! sinuses faint. 



Brachial valve with a marginal beak, which is much thickened 

 in some instances, without producing a cardinal area, though 

 making a prominent apophysis which is extended against the 

 cardinal slope of the opposite valve. Crescent as in the other 

 valve. Platform higher than in the pedicle-valve, its vaults 

 extending scarcely so far backward. Anterior median septum 

 long, often reaching almost to the margin. Muscular and pallial 

 impressions faint. 



Type, Trimerella grandis, Billings. 



Distribution. Upper Silurian. (Illinois, Wisconsin, Ohio, 

 Province of Ontario, Gotland, Faro.) Five species are known in 

 American faunas and two additional species occur in the Swedish 

 rocks. 



Dinobolus, Hall. 1871. 



(Plate 2, Figs. 1-4.) 



Synonyms; Conradia, Hall, 1862; Obohellina, Billings, 1871. 



Shell sub-circular ; valves convex, thick. 



Pedicle- valve with an acute and slender beak ; cardinal area 

 triangular, more or less elevated, but never so high as in Trimer- 

 ella or Monomerella. Cardinal margins sometimes of considerable 

 width. Umbo generally solid, but often slightly excavated on 

 either side of a simple or double vertical wall, or buttress. Cres- 

 cent distinct, its apex making a retral angle; narrow over the 

 crown, widening into the great terminal scars. Platform low, 

 broadly V-shaped on its anterior margin, which usually lies at 

 about the middle of the valve ; generally, with very short, flat- 

 tened, abruptly conical vaults, but occasionally these vaults are 

 of the same relative length as in Trimerella, though much shal- 

 lower. Central and lateral muscular scars on the platform usually 

 distinct, the former lying in a depressed median groove, the latter 

 occupying the greater portion of the surface. Subcardinal and 

 anterior scars rarely discernible. Anterior median septum scarcely 

 developed. Pallial sinuses faint. 



Brachial valve with the beak submarginal, inconspicuous, usually 

 not discernible ; hinge-area very narrow. Crescent very strong, 

 1891 70 105 



