Report of the State Geologist. ,")89 



developed. On the interior the pedicle*valve bears strong 

 teeth, highly developed convergent dental Lamellae, and an 

 elevated median septum which extends to the middle of the 

 valve, terminating abruptly. In the brachial valve the crura are 

 recurved over the cardinal line and are supported by strong 

 septa: cardinal process small, erect and multilobate. Muscular 

 scai-s very faint. Shell substance punctate, tubulose at the 

 surface. 



{i Jarboniferous.) 



Billingselia, Hall. L892. 



(Plate 8. Fi£s. l. -J.) 



Shell transverse; subquadrate or semicircular in outline. Con 

 tour concavo — , or plano-convex. Surface sharply striate or j dicate. 

 Pedicle-valve the more convex; cardinal area moderately high, 

 vertical or slightly incurved. Delthyrium covered by a convex 

 plate, which, in rare instances, may be minutely perforated at 

 the apex ; the teeth are well developed, but the dental plates are 

 continued along the bottom of the umbonal cavity, only near 

 the apex inclosing a small subelliptical muscular area. In the 

 brachial valve the cardinal area is greatly inclined, making an 

 obtuse angle with that of the opposite valve. The delthyrium 

 is partially covered by a convex deltidium which never attains 

 the development seen in the opposite valve, and is often wholly 

 absent. 



Type. Orthis Pepma, Hall. 



Distribution. Cambrian — (?) Upper Silurian. 



Protorthis, Hall. 1892. 



(Plate 8. Figs 3-7.) 



Shells small, transversely subquadrate or semicircular. Hinge- 

 line straight, its length being equal to the greatest width of the 

 valves. Valves unequally biconvex, or sub-planoconvex, the 

 pedicle-valve being the larger. The cardinal area is narrow on 

 both valves, but is higher on the pedicle-valve, and is transected 

 by a broad delthyrium which is closed below by a concave plate 

 apparently produced by the union of the dental lamella 1 , which 

 are not continued to the bottom of the valve ; teeth distinctly 

 developed. In the brachial valve the cardinal area also bears an 

 open delthyrium ; the dental sockets are obscure and the crural 



141 



