94 
PALAEONTOLOGY OF NEW YORK. 
sinuate valves are distinctive features and to these must be added the peculiar 
character of the muscular scars. This subtjpe is largely represented in the 
American Carboniferous faunas and from an examination of its various species 
we deduce the following characterization; 
Shells transverse, often elongate or ficiform; valves biconvex, the pedicle- 
valve with a median sinus over the pallial region, and the brachial valve with a 
corresponding ridge; both sinus and fold may be divided by a sharp median 
sulcus extending from the umbones to the margins. There is frequently evi¬ 
dence of a single obscure lateral fold on each side of both valves. The umbo 
of the pedicle-valve is incurved and the deltidial area is usually concealed; the 
foramen, however, is exposed as a circular or ovate aperture which encroaches 
on the substance of the valve. In the pedicle-valve the diductor muscular im¬ 
pressions are very faintly defined; the adductor and pedicle ipipressions are as 
in the typical forms of Athyris. In the brachial valve the hinge-plate is highly 
developed, its upper face being subquadrate in outline, concave on the surface, 
the concavity deepening toward the visceral foramen which lies just beneath 
the beak; not infrequently the foramen is closed by secretions of testaceous 
matter. The posterior flanges of the plate pass beyond the hinge-line and into 
the urnbonal cavity of the opposite valve. The anterior face of the plate is 
erect and the anterior edge somewhat trilobed, the lateral lobes bearing the 
crural bases. The crura are straight and their attachment to the primary 
lamellae is of the same character as in Cliothyris, etc. 
The primary lamellae, on the urnbonal curve, are broad, 
the loop usually situated posteriorly. The saddle of the 
loop is often bilobed on its anterior margin, and frequently 
both it and the outer margins of the ribbon of the sec¬ 
ondary volutions are fimbriated. 
The muscular impressions of this valve are very narrow, and subdivided into 
two pairs of elongate scars. The members of the posterior pair are divided 
by a median septum or ridge, which begins beneath, though it does not support 
the hinge-plate. Branching vascular sinuses are sometimes retained over the 
pallial region of both valves. 
Fig. 65. 
Loop of Athyris trinuclea, 
Hall. .bt. Louis liioe- 
stone. (C.) 
