BraonioroDA. W717 
In the brachial valve the hinge-plate is supported by a median 
septum slightly longer than that of the opposite valve, and the 
narrow, cordate muscular impression, which it divides medially, 
is considerably thickened. The valve bears everted spirals 
similar to those of other members of this group, but the speci- 
mens studied were not in a condition of preservation adapted to 
the determination of the structure of the jugum. 
Type, Camarospira Eucharis, Hall. Corniferous limestone. 
Athyris, McCoy. 1844. 
(Plate 35, figs. 1-8.) 
Synonyms; Spirigera, D’Orbigny, 1847; Huthyris, Quenstedt, 
1871. 
Shells subequally biconvex; outline transversely elliptical, 
subcircular or elongate-subovate ; surface medially sinuate. 
In the pedicle-valve the umbo is inconspicuous and incurved, 
usually concealing the foramen and deltidial plates; frequently, 
the former is exposed. Cardinal slopes not well defined in the 
typical group. The convexity of the valve is greatest in the 
umbonal region, the surface sloping evenly to the sides, and 
becoming depressed on the median line into a sinus, which is 
most prominent on the anterior margin. Beak of the brachial 
valve not prominent; a median fold corresponds in strength to 
the sinus of the opposite valve. 
In the interior of the pedicle valve the deltidial plates are 
usually absent; the teeth are prominent, recurved at the tips, 
and supported by stout dental lamelle, which are not produced 
anteriorly about the muscular area. Between them lies a deep, 
transversely striated pedicle-cavity, and in front of this an ovate 
muscular scar extending about one-half the length of the valve 
and divided into flabellate diductors (which are frequently very 
indistinct) and narrow, cordate adductors. The pallial region 
is covered with ovarian pittings and branching sinuses. 
In the brachial valve the dental sockets are broad and deep. 
The hinge-plate varies considerably in form; in the typical divi- 
sion of the genus it is subtriangular in outline, and supported by 
stout crural plates. The median portion is flat or concave, the 
lateral margins thickened and elevated. At the apex of the plate 
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