816 Report oF THE State GEoLoGIsT. 
but three or four volutions. The jugum is situated posteriorly 
and constructed as in Arrypa, except that its lateral lamelle 
appear to be always united in an acute angle, which is directed 
inward. 
Muscular impression composed of large flabellate diductors, 
enclosing distinct adductor scars. 
Type, Airypina imbricata, Hall. 
Distribution. Upper Silurian —Devonian. 
Atrypa, Dalman 1828. 
(Plate 41, figs. 7-17.) 
Shellssubcircular or longitudinally subovalin outline. Gibbous, 
strongly inequivalve. Hinge-line short, straight; cardinal 
extremities rounded. Beaks not prominent. 
Pedicle-valve the smaller; convex in the umbonal region, but 
depressed and often deeply sinuate anteriorly. Beak small, 
usually incurved in advanced growth-stages, concealing the 
foramen and deltidium, The foramen is triangular in young 
shells, extending to the hinge-line, but becoming gradually closed 
by the growth of deltidial plates, and at maturity is circular and 
apical, encroaching slightly on the substance of the valve. The - 
plates of the deltidium are not coalesced along the median suture. 
On the interior the umbonal cavity is short but very broad. The 
teeth are large, widely separated and doubly grooved, first by an 
oblique furrow at the base, into which is fitted a crenulated ridge 
of the other valve, then by a short longitudinal depression on the 
summit; the tooth is doubly curved and reflected, making the 
articulation of the valves very firm. These teeth arise from the 
inner surface of the lateral slopes of the valve, and are hence 
unsupported by lamellz. The muscular impressions are sharply 
defined ; the triangular pedicle-scar is followed in front, by a 
median elongate double scar of the adductors, outside of which 
are strong, radiately striate, flabellate diductors, which frequently 
extend beyond the middle of the valve. 
Brachial valve convex or rotund in the middle, with a median 
fold which is rarely developed except toward the anterior margin. 
Beak incurved and concealed. No cardinal area. The hinge- 
plate is composed of two diverging processes which may or may 
not meet at the apex. Each of these processes is obliquely 
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