Report of tee State Geologist, 607 



rather regular concentric plications. In the pedicle-valve the 

 cardinal area is moderately high and the delthyrium in Its normal 

 condition probably closed by a convex plate. The teeth are very 

 divergent and from their bas n<l the elevated margins of 



two linguiform muscular scars, traversing the shell for almost its 

 entire Length. These scars, which may be regarded as the diduc- 

 torSj inclose two. miu-li shorter, but still elongate adductors. In 

 the brachial valve the cardinal process is bipartite on its anterior 

 face, each of the lobes being grooved behind; tin' crural plates 

 are verv Long and divergent, terminating in elevated extremities 

 or crura. The lower moiety of these plates is produced on each 

 side of a strongly elevated muscular ridge, curving slightly 

 inward on the side-, then outward on approaching the anterior 

 margin of the valve, eaeh branch recurving and passing backward, 

 parallel to the median axis, as far as the base of the cardinal pro- 

 The symmetrical spaces thus limited are each divided trans- 

 versely at about one-third their length from the hinge-line, by a 

 somewhat lower vertical ridge. The four areas thus inclosed rep- 

 resent the posterior and anterior scars of the adductor muscles. 

 Between the inner muscular walls, in the median line, is a low, 

 rounded longitudinal ridge. 



Type, Christiana mbquadrata, Hall. 



Distribution. Lower Silurian (?) — Devonian. 



Leptasnisca, Beecher. 1800. 

 (Plate 19, Figs. 14-21.) 



Shell concavo-convex, attached to foreign objects by calcareous 

 cementation of the pedicle-valve. Brachial valve concave ; inte- 

 rior with a broad, more or less defined, spiral impression on each 

 side of the median line, making a single volution. Adductor 

 impressions small. Cardinal area narrow, bearing in the center 

 two prominent bilobed, cardinal processes, separated to admit the 

 vertical septum in the opposite beak. Pedicle-valve convex, area 

 elongate triangular, fissure covered with a pedicle-sheath. Car- 

 dinal muscular scar supported on, or limited by a vertical septum, 

 on each side of which, in the anterior half, is a small adductor 

 scar. Shell structure punctate. 



Type, LepUenisca concava, Hall (sp.h 



Distribution. Lower Devonian. 



159 



