582 Forty-fifth Report on the State Museum. 



an elongate, vertical plate, extending from the apex the whole 

 length of the delthyrium, thus longitudinally dividing the deep 

 deltidial cavity. It is usually simple, both on the outer edge and 

 at its distal extremity. Shell structure compactly fibrous and 

 impunctate, except for large oblique tubules penetrating the 

 external layers. (Lower — Upper Silurian.) 

 Plectorthis, Hall. 1892. 

 II. Group of Orthis plicatella, Hall. 

 (Plate 9, Figs. 8-10.) 

 Yalves subequally convex. Cardinal area of pedicle- valve low ; 

 plications strong, simple or duplicate. Shell substance impunc- 

 tate. (Lower Silurian — (?) Lower Devonian.) 



Dinorthis, Hall. 1892. 



III. Group of Orthis pectinella (Conrad, MS.), Emmons. 



(Plate 9, Figs. 11-15.) 



Relative convexity of the valves reversed. Surface with strong, 



rarely bifurcating plications. Delthyrium sometimes partially 



closed. Muscular area in pedicle-valve subquadrate, bounded by 



extensions of the dental lamellae. In the brachial valve the 



muscular impressions are obscure ; cardinal process erect, broad 



and frequently bilobed on its posterior face. Shell structure 



impunctate. (Lower Silurian.) 



Plaesiomys, Hall. 1892. 

 IV. Group of Orthis subquadrata, Hall. 

 (Plate 9, Figs. 16-20.) 

 Valves with reversed convexity. In the interior of the pedicle- 

 valve there is a large quadrate muscular scar, with a tendency to 

 bilobation by the lateral extension of the diductor impressions. 

 The subdivision of this area into separate scars is frequently very 

 distinct. In the brachial valve the cardinal process is thickened 

 and erect at its posterior extremity, being distinctly crenulate on 

 the summit and posterior face. The surface striae are tubidose 

 and the shell-structure fibrous-impunctate. (Lower Silurian.) 

 Hebertella, Hall. 1892. 

 V. Group of < >rthis sinuata. Hall. 



(Plate 10, Figs. 1-4.) 

 Shells with convexity of the valves reversed. Surface covered 

 with numerous lino, rounded, closely-crowded plications, crossed 

 by lamellose- growth-lines. On the interior of the pedicle-valve 



134 



