854 Report or THE State Geo oaist. 
The muscular impressions occupy an elongate area below the 
hinge-plate, and are divided by a median ridge, but are only 
obscurely divisible into their elementary scars. The lateral por- 
tions of this valve frequently bear a series of vascular sinuses in 
the pallial region. 
Surface smooth or with concentric lines crowded near the 
margins of the valves. Shell-substance punctate. 
Type, Centronella glans-fagea, Hall. Upper Helderberg 
group. 
Distribution. Devonian. 
Subgenus Oriskania, Hall. 1893. 
(Plate 52, figs. 11-13.) 
Shells large with the characteristic naviculoid form and smooth . 
exterior of Cenrronetia. The form of the brachidium has not 
been determined, but there is every reason to infer that it differs 
in no essential features from that of Cemironsitia. The hinge- 
Fic. 463. Fie. 464. Fia. 465. 
Oriskania navicella, Hall. 
Fie. 463. A cardinal view; showing the hinge-plate and the elevation of the narrow median 
crest or cardinal process. The specimen also retains the teeth of the pedicle- 
valve and a portion of the dental lamelle, though the latter are broken pear the 
surface of the valve. 1 
Fie. 464. An enlargement of the hinge-plate; showing the crural lobes and the extent of the 
cardinal process. 
Fie. 465. A profile of the same specimen; showing the thickness of the plate and the uncinate 
form of the cardinal process. x 8. 
plate is elongate-triangular, continuous between the crural bases, 
and bears a median vertical crest, or cardinal process, which 
begins at the apex, rises rapidly in height, and extends for fully 
one-half the length of the plate on its upper edge, but at its base- 
106 
