Report or the State Geologist, 597 



Bometimes fasciculate striaB; rarely smooth. Shell-substance 

 fibrous, coarsely punctate. 

 Type, Stroj ■ lonrad (sp. i. 



Distribution. Lower Silurian I f) — Dpper Devonian. 



Subgenus Pholidostrophia, Ball. l v 

 (Plate 1 1. Pigs. 1; 

 Shells small ; surface smooth or squamous ; nacreous. Interior 

 of the brachial valve with three diverging ridges in front of the 

 muscular area. (Type, P. nacrea, Hall (sp.). Devonian.) 



Subgenus Leptostrophia, Hall. 1892. 

 (Plate 15, Figs. 1-4.) 

 Shell plano-convex or flat; delthyrium more or less completely 

 closed. In the pedicle-valve are two pnstulose diverging ridges, 

 bounding the sides of the muscular area. Cardinal scars broad 

 and Habellate; not distinctly limited anteriorly. (Type. L. mag- 

 )ih'ra< Hall. Devonian.) 



(?) Subgenus Brachyprion, Shaler. \^:k 

 (Plate 14, Figs. 7-10.) 

 Shells with delthyrium open or partially covered ; crenulations 

 of the cardinal margin confined to a very limited extent on either 

 side of the fissure. 



(Type, B. Leda, Billings (sp.). Upper Silurian.) 



Subgenus Douvillina, (Ehlert. 1887. 

 (Plate 14, Figs. 16-19.) 



Shells having a crenulated hinge and flat deltidium. In the 

 pedicle-valve are two small teeth connected with diverging 

 ridges which are strongly elevated in front, and inclose a trans- 

 verse muscular area. Beneath the beak 

 are two crests supported by a short 

 median septum; posteriorly these all 

 unite to form a single apophysis grooved 

 in the middle and uniting with the 

 inner surface of the deltidium. The 

 brachial valve has a prominent bifid 

 cardinal process, and a pair of small ^o. a7o:_^ST^e of Dauvii 

 protuberances representing the crural una Dwtertrn. After cehlert. 

 plates; from the base of the cardinal process extends a median 



149 



