Report of the State Geologist* 583 



the teeth are large and supported by thick lamellae which ore 

 continued as a strong ridge around a short obcordate muscular 

 area. This area is medially divided by a prominent ridge upon 

 the summit of which lies the linear scar of the adductors. The 

 flabellate lateral impressions are sometimes divisible into their 

 two components, diductors and adjusters, and in old individuals 

 the impression of the pedicle-muscle is often distinct. 



In the brachial valve the dental sockets are narrow and are 

 inclosed beneath, and on the inner side, by the strong crural 

 plates. The cardinal process is elongate and simple, sometimes 

 thickened, often crenulate, but not lobed at its posterior 

 extremity. The process unites with the inner basis of the crural 

 plates and is produced forward as a median ridge dividing the 

 four muscular scars, which are distinctly developed only in old 

 shells. 



Shell-structure fibrous-impunctate ; the plications of the surface 

 sometimes tubulose. 



(Lower — Upper Silurian.) 



Orthostrophia, Hall. 1883. 

 VI. Group of Orthis strophomenoides, Hall. 

 (Plate 10, Figs. 5-8.) 



Contour and surface as in Hebertella. In the interior of the 

 pedicle- valve the muscular area is deeply excavated, and limited 

 almost to the narrow space between the dental lamella?. The 

 adductor scars are extremely minute and linear, while the diduc- 

 tors occupy the rest of the area. In the opposite valve the mus- 

 cular area is similarly confined but the quadrilobate character of 

 the impression is very decided. The lateral components are 

 divided by a strong median ridge, and the entire area is deeply 

 impressed in the substance of the shell. The cardinal process is 

 elongate and simple at its posterior extremity, the crural plates 

 and crura strong. 



The vascular and ovarian markings are conspicuous. In the 

 pedicle-valve two or three large vascular trunks originate near 

 the anterior edge of the muscular area, divide a few times in 

 their passage over the pallial region, the branches rapidly multi- 

 plying near the margin. In the opposite valve the main sinuses 

 are four in number, originating in pairs at the ante-lateral mar- 



135 



