814 Report or THE STATE GEOLOGIST. 
Subgenus Orthonomea, Hall. 1850. 
(Plate 40, figs. 34-87). 
Large shells, having the contour of Zyeospira, but with ex- 
tremely fine surface plications. 
(Type, Orthonomea erratica, Hall. Hudson River group.) 
Clintonella, Hall. 1893. 
(Plate 40, figs. 38-44.) 
Shells usually small, suboval in outline; valves subequally 
biconvex, the axis of greatest convexity being oblique, making 
an angle of about fifty-five degrees with the vertical axis of the 
shell. Pedicle-valve with a small umbo, which is compressed 
laterally, the apex being slightly incurved. The cardinal area is 
replaced by a wide triangular delthyrium, which is unaccom- 
panied by any trace of deltidial plates. The medially elevated 
umbo merges anteriorly into a sinus which makes a deep flexure 
at the margin ; it bears two plications, both of which reach the 
ocak; sometimes a trace of a third plication may be seen. The 
lateral slopes bear from four to eight radial plications of 
smaller size. 
On the interior the teeth are prominent, strongly recurved at 
their tips and supported by lamellae which terminate abruptly. 
The lower and inner margins of these lamelle are thickened, 
contracting the pedicle cavity, which is, consequently, narrow 
and deep. The diductor scars are of moderate size, flabellate in 
outline and deeply impressed at their posterior extremity. They 
are crossed by traces of the radial surface plications. Between 
them lie the narrow obovate adductor scars. 
In the brachial valve the beak is inconspicuous; the umbonal 
region depressed for about one-third the length of the shell, 
thence anteriorly becoming developed into a median fold. The 
greatest convexity of the valve is attained in front of the center. 
The cardinal margin is scarcely thickened; the dental sockets 
quite narrow. The hinge-plate consists of two flattened pro- 
cesses, inclined toward each other and closely approximate along 
their inner bases, though not meeting. Each process is divided 
into an anterior and posterior lobe, the latter being the smaller 
and resting upon the former. These anterior lobes are narrow 
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