830 Report or THE State GEOLOGIST. 
and in front of them a larger pair whose surface is radiately 
striated, the entire area being elongate-oval. Vascular impres- 
sions are occasionally retained in the pedicle-valve. 
Type, Hatonia medialis, Vanuxem (sp.). 
Distribution. Lower Devonian. 
Cyclorhina, Hall. 1898. 
(Plate 42, figs. 27-31.) 
Shells of comparatively large size at maturity, subtriangular in 
outline; biconvex, the convexity of the brachial valve being the 
greater. Fold and sinus very broad, and developed in the usual 
manner, on brachial and pedicle-valves respectively. 
On the pedicle-valve the apex is obtuse, not elevated, and is 
very broadly truncated by a large circular foramen, which, even 
in the earliest growth-stages observed, is enclosed for fully five- 
sixths of its periphery by the substance of the valve. The del- 
tidial plates are incipient at maturity and scarcely evident in 
young shells; the delthyrial margins are extremely divergent. 
The cardinal line is short but straight, and its extremities are 
produced on each side to form a short alate process or wing, 
similar to those in the genus Eumerrta. These extensions occur 
on both valves, and are very apparent in the younger shells, but 
become somewhat obscured with the increase of convexity accom- 
panying maturity. On the interior, the teeth are large and. 
blunt, and attached to the lateral walls of the shell, though they 
also rest upon the thick lamellz similarly attached except 
at their anterior margins, and which converge downward to 
form a deep, broad, transversely striated pedicle-cavity. The 
thickened lateral margins of this impression are continued 
anteriorly to about the center of the shell, forming an elongate- 
quadrate diductor scar which incloses a small oval adductor. 
The brachial valve has a convex umbo, showing no evidence of 
concavity in early stages of growth. Beneath the beak is a very 
fine, vertical, linear cardinal process which appears to be contin- 
uous with an obscure median longitudinal ridge, traversing about 
one-half the length of the valve. Both of these are frequently 
involved in the shell-substance and evident only in sections of the 
shell. The hinge-plate is deeply divided medially, each lateral 
portion being supported by a deep vertical septum resting on the 
bottom of the valve. The upper surfaces of the hinge processes 
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