592 Forty-fifth Report on the State Museum. 



Scenidium, Hall. 1860. 

 (Plate 8, Figs. 21-27.) 



Synonym ; Mystrophora, Kayser, 1871. 



Shell subpyramidal, somewhat semicircular, with or without 

 median sinus and elevation. Pedicle-valve elevated, subpyra- 

 midal; beak straight or slightly arched; muscular impressions 

 undetermined. Cardinal area large, triangular, divided by a 

 narrow fissure, which is sometimes closed at the summit by a 

 concave plate. Brachial valve flat, or varying from depressed- 

 convex to concave. Beak entire, or indented by the foramen ; 

 cardinal line straight and usually equaling the width of the shell ; 

 crural plates broad and well developed, marked by the imprints 

 of the pedicle muscles, and produced in the middle in a pointed 

 process ; the cardinal process extends, as a median septum, 

 through the length of the shell, and may be simple or divided 

 at its anterior extremity. Exterior surface covered with radiat- 

 ing stride. 



Type, Scenidium insigne, Hall. 



Distribution. Lower Silurian — Middle Devonian. 



Orthidium, Hall. 1892. 

 (Plate 8, Figs. 28-31.) 



Shell very small, transverse, having the external aspect of 

 Scenidium. Hinge-line long, making the greatest diameter of 

 the shell. Pedicle-valve the more convex ; cardinal area moder- 

 ately high, with a broad open delthyrium, strong teeth and 

 inconspicuous dental plates ; in all respects like the correspond- 

 ing valve of Orthis calligramma. Brachial valve slightly convex ; 

 cardinal area very narrow; dental sockets well developed, crural 

 plates Very short and erect, coalesced with the cardinal process, 

 which thus becomes a vertical, transverse, subcrescentic plate, at 

 the base of which the shell is somewhat excavated ; muscular scar 

 quadruplicate. Surface covered with radiating striae, which 

 extend over the broad, low sinus and fold in the brachial and 

 pedicle- valves respectively. In the former the sinus makes a 

 prominent median ridge in the interior of the valve. Shell- 

 structure not determined. 



Ty|>< k , Orthidium gem/macula^ Billings. 



(Lower Silurian.) 



IN 



