268 
PALEONTOLOGY OF NEW YORK. 
Shells plano-convex or concavo-convex. Pedicle-valve with acute incurved 
beak, perforated at its extremity, the foramen being continuous with a partially 
closed delthyrium; medially ridged, and with abruptly sloping sides. On the 
interior the teeth are large, thick at their extremities and adherent to the 
lateral walls of the shell. Between them is a deep pedicle-cavity, in the bottom 
of which lie the elongate scars of the adductor muscles, and about their ante¬ 
rior portion the small, flabellate diductors. 
The brachial valve is very shallow, rendered concave exteriorly by a median 
sinus which does not make itself apparent on the interior. Beak small, apex 
not incurved. Dental sockets broad, bounded interiorly by the high walls of 
the hinge-plate. This plate is divided medially by a deep furrow extending to 
the apex, and therefore consists of two processes which are elevated, thickened 
and rest on the bottom of the valve. From the anterior face of these arise 
the crura which converge for a short distance, and 
expand to form two broad acute jugal processes. 
From here the lateral branches of the brachidium 
curve outward, gradually turning from a vertical to 
a horizontal position, broaden rapidly and unite to 
form an anterior triangular plate which bears a 
median ridge, where the two lateral branches are 
conjoined. The whole of the anterior portion of the brachidium is inclined 
gently upward toward the cavity of the opposite valve. 
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 portions 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, Rhynchonella glans-fagea, Hall. Upper Helderberg group. 
Fig. 180. 
Centronella glans-fagea, Hall. 
A preparation showing the hrachidium. 
(Romingeu ) 
Observations. The type of external form and internal structure exemplified 
by this species is probably also represented by the C. alveata, Hall {—C. Hecate, 
