Braowiopopa. 779 
inclined downward or toward the beak of the brachial valve for 
a short distance, thence it rises abruptly toward the umbo of the 
pedicle-valve, and bifurcates near the extremities of the crura, 
each branch following the curvature of the primary lamelle and 
continuing for only a part of the distance between the ends of 
the crura and the origin of the jugum. These accessory lamelle 
vary somewhat in form, are narrower than the ribbon of the coil, 
and lie between the primary, and the first band of the secondary 
lamellee. 
The muscular area consists of a long, ovate scar, which is 
divided into a subquadrate posterior pair, and a subcordate ante- 
terior pair of adductor impressions. These are separated longi- 
tudinally by a very faint median ridge. On casts of the interior 
the filling of the visceral foramen in the hinge-plate frequently 
shows a cross-striation like that of the pedicle-cavity of the oppo- 
site valve, and also indicates that the median ridge is continued 
throughout the extent of this passage. 
The surface of the valves is variously ornamented; in the typi- 
cal group, at each concentric growth-line, there is a broad lamellar 
expansion ; in some cases this expansion is striated longitudinally, 
or it may be divided into flat spines, which merge into the 
lamella at their bases ; again the spines may be long and tubular, 
but connected by the laminar expansions. The surface frequently 
appears to be smooth or covered only with concentric striw, and 
in one of the largest subdivisions of the genus (Srmmura) it is a 
normal condition, while in other divisions it is often wholly 
casual. 
Shell-substance fibrous, impunctate. 
Type, Athyris concentrica, von Buch (sp.). Middle Devonian. 
Distribution. Upper Silurian (?) — Carboniferous. 
Subgenus Cliothyris, King. 1850. 
(Plate 35, figs. 9, 10.) 
Shells with surface ornamentation consisting of broad, thin, 
lamellar expansions, which are divided almost, sometimes quite 
to their bases into long, flat spinules; the hinge-plate is narrow 
and rather acutely triangular ; the primary lamellz are attached 
to the crura not only at their apices but fora short distance along 
31 
