584 Forty-fifth Report on the State Museum. 



gins of the anterior adductors. The outer member of each pair 

 curves quite abruptly toward the cardinal angle, while the inner 

 members] curve outward and then inward, all ramifying as they 

 approach the margins of the shell. The unibo-lateral spaces 

 inclosed by the curvature of these sinuses in both valves, and the 

 central space in the brachial valve, are covered with linear den- 

 dritic ovarian striae. Shell substance impunctate. 

 (Lower Devonian.) 



Platystrophia, King. 1850. 



VIE. Group of Orthis biforata, Schlotheim (sp.). 

 (Plate 10, Figs. 9-13.) 



Contour spiriferoid; hinge-line and area long and straight, 

 nearly equally developed on the two valves. Both valves very 

 convex, the brachial being the more so, and bearing a very strong 

 median fold corresponding to a deep sinus on the opposite valve. 

 Surface of valves marked by strong, sharp plications, which extend 

 over the fold and sinus, and the external surface is finely granu- 

 lose, the latter feature being rarely well retained. Delthyrium 

 open in both valves, being somewhat larger in the pedicle-valve, 

 and in old and gibbous shells of Orthis lynx it has often encroached 

 to a considerable extent upon the umbonal region of the valve. 

 Teeth thick and very prominent ; muscular area comparatively 

 small, but usually deeply excavated in the substance of the shell, 

 and not readily divisible into the component scars. The cardinal 

 process of the brachial valve is a simple linear ridge, always small 

 and sometimes nearly obsolete. Dental sockets comparatively 

 small ; crural plates large and thick, uniting at their inner bases 

 and produced into a prominent median ridge. Muscular area 

 quadruplicate. Shell-structure very compact and finely fibrous, 

 without punctation. 



(Lower — Upper Silurian.) 



Heterorthis, Hall. 1892. 



VIII. Group of Orthis Clytie, Hall. 

 (Plate 11, Figs. 1-3.) 



Contour leptaenoid. Shell transversely oval, with a straight 

 binge-line and rounded cardinal extremities; its form is depressed 



planoconvex, the convexity of the pedicle valve being slight. 



136 



