608 Forty-fifth Report on the State Museum. 



Davidsonia, Bouchard. 1849. 



(Plate 19, Figs. 22-24.) 



Shell thick, plano-convex, transversely oval. Surface smooth 

 or with concentric growth-lines, fixed to foreign bodies by the 

 umbonal portion and the greater part of the surface of the 

 pedicle-valve. Hinge-line straight and quite narrow. On the 

 pedicle-valve the delthyrium is covered by a convex imperforate 

 plate ; the teeth are large, the dental lamellae obscure. The 

 muscular area is comparatively small, lying in the umbonal 

 region, and is subdivided into two cardinal scars inclosing an 

 elongate adductor. In the pallial region there is a low median 

 septum which separates two conical callosities of the shell, 

 having their apices directed toward the opposite valve. These 

 protuberances are grooved by a spiral furrow which makes five 

 or six volutions, and are frequently crossed by vascular sinuses. 



In the brachial valve the chilidium is convex, embracing 

 the base of the posterior face of the cardinal apophysis. The 

 cardinal process has very much the same structure as in Plectam- 

 bonites ; consisting of a central, short, erect process, to which the 

 crural plates are attached, giving it a trilobate appearance. 

 These plates terminate abruptly at their distal extremities. The 

 muscular area is quadruplicate and of about the same size as in 

 the opposite valve. Two conical depressions in the pallial region 

 correspond to the protuberances of the opposite valve. Shell- 

 substance punctate ( ?). 



Type, Davidsonia Verneuili, Bouchard. 



Distribution. Middle Devonian (Europe). 



Chonetes, Fischer de Waldheim, L837. 

 (Plate 20, Figs. 1-14.) ' 



Shells semicircular or transverse, usually normally concavo- 

 convex, sometimes plano-convex. Hinge-line straight, making 

 the greatest diameter of the shell. Pediole-valve with a narrow 

 concave or' flat cardinal area; the delthyrium more <>r less com- 

 pletely covered by a convex imperforate deltidium. The upper 

 margin of the area boars a single row o\' hollow vertical or 



divergent spines, which increase in Length toward the cardinal 



angles; these spines are the prolongations of tubes which 



lt;o 



