870 Report oF THE STATE GEOLOGIST. 
fhell-substance impunctate externally, but the inner laminz 
are sparsely perforated. 
Type, Stringocephalus Burtini, Defrance. Middle Devonian. 
Tropidoleptus, Hall. 1857. 
(Plate 54, figs. 1-11.) 
Shells with the general external aspect of RarmEsquina ; con- 
cavo-, or plano-convex. Hinge-line straight; in young shells 
forming the greatest transverse diameter and frequently extended 
at the cardinal extremities, but in mature and old shells shorter » 
than the transverse diameter in the pallial region. Marginal 
outline varying from longitudinally semi-elliptical in youth, to 
transversely subelliptical at maturity. Surface covered with 
simple, low plications, all extending from beak to margins. The 
median plication on the pedicle-valve and the corresponding 
sinus on the brachial valve are broader and more conspicuous 
than the others. 
The pedicle-valve is regularly convex, becoming slightly con- 
cave on the cardinal slopes. It bears a moderately broad cardi- 
nal area, coextensive with the hinge-line, which is divided by a 
broad, open delthyrium, which, in no observed condition of 
growth, bears a covering of any sort, but is filled by the cardinal 
process of the other valve. The base of the delthyrial cavity is 
thickened and transversely striated, probably by the attachment 
of the pedicle-muscle. The teeth are not situated at the extremi 
ties of the delthyrial margins, but lie within and in front of them, 
arise from the bottom of the valve as two erect, divergent sub- 
quadrate crests, resting upon low ridges which bound the muscu- 
lar area. These peculiar teeth are smooth and abrupt on their 
inner faces, while their outer faces are deeply crenulated. A 
low groove separates each from the cardinal area. The muscular 
area is broadly flabellate, extending more than half-way across 
the valve, and consists of two large diductor scars enclosing a 
narrow median pair of adductors. 
The brachial valve is slightly concave, often nearly flat. Cardi- 
nal area narrow, but clearly developed; chilidium prominent. 
Cardinal process large, erect, smooth on its posterior surface, and 
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