302 PALAEONTOLOGY OF NEW YORK. 



Diagnosis. 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 delthyriuni is covered by a con- 

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

 muscular area is comparatively small, lying in the umbonal region, and is sub- 

 divided into two cardinal scars enclosing 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 pro- 

 tuberances are grooved by a spiral furrow which makes five or six volutions, 

 and are frequently crossed by vascular sinuses. 



In the brachial valve the deltidial covering is convex, embracing the base of 

 the posterior tace of the cardinal apophysis. The cardinal process has very 

 mucli the same structure as in Plectambonites, consisting ol 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 protuber- 

 ances of the opposite valve. Shell-substance punctate (?). 



Type, Davidsonia Verneuili, Bouchard. Middle Devonian. 



Observations In 1859, de Koninok detected the existence of calcified spiral 

 brachial supports in this genus, which would give it somewhat the character of 

 Atrvi'A. Dr. (Ehlert has placed the genus with some doubt in the family 

 KoifiNCKJifiij^, with Koninckina, Anoplotheca, Koninckella, Amphiclina, The- 

 cospira; the last of these genera being similarly attached by the surface of tlie 

 pedicle-valve, and all of them being spirigerous. 



Davidsonia occurs in the middle Devonian (Crinoiden-schichten) of the Eifel, 

 and at an essentially equivalent horizon in England, Belgium and Russia. But 

 two species have been described, D. Verneuili, Bouchard, and D. Bouchardiana, de 

 Koninck, the latter being regarded by P. Roemer and Kayser as identical with 

 the former. The genus is not represented in American faunas, so far as known. 



