BRACHIOPODA. 
37 
Diagnosis. Shell sub-circular; valves convex, thick. 
Pedicle-valve with an acute and slender beak; cardinal area triangular, more 
or less elevated, but never so high as in Trimerella or Monomerella. Cardinal 
margins sometimes of considerable width. Umbo generally solid, but often 
slightly excavated on either side of a simple or double vertical wall, or buttress. 
Crescent distinct, its apex making, a retral angle; narrow over the crown, 
widening into the great terminal scars. Platform low, broadly V-shaped on 
its anterior margin, which usually lies at about the middle of the valve; 
generally, with very short, flattened, abruptly conical vaults, but occasionally 
these vaults are of the same relative length as in Trimerella, though much 
shallower. Central and lateral muscular scars on the platform usually distinct, 
the former lying in a depressed median groove, the latter occupying the greater 
portion of the surface. Subcardinal and anterior scars rarely discernible. 
Anterior median septum scarcely developed. Pallial sinuses faint. 
Brachial valve with the beak submarginal, inconspicuous and usually not 
discernible; hinge-area very narrow. Crescent very strong, especially over 
the crown, where it makes a sharp posterior angle beneath the beak, 
sloping thence very gently, being almost transverse for about the width 
of the platform, then bending quite abruptly and being slightly interrupted at 
the position of the terminal scars, which are more approximate than in the 
opposite valve. In the type-species, the crown of the crescent, as usually 
developed, is a sharply incised, narrow furrow, bounded within by a broad ele¬ 
vation sloping to the position of the subcardinal scar, but not infrequently it 
is a ridge, bounded in front and behind by a deep furrow. This difference in 
appearance is due to the varying development of the anterior furrow, which is 
sometimes quite suppressed. Platform narrower and more sharply V-shaped 
than in the opposite valve; vaults usually very short and abruptly conical. 
Lateral and central scars on the platform conspicuous, the latter lying in a deep 
median furrow, at the front end of which are sometimes seen the faint anterior 
scars. In the line of this furrow, directly in front of the apex of the crescent, 
lies the deep impression of the subcardinal muscle, which makes a prominent 
feature on the cast, suggestive of the filling of the umbonal cavity, which it 
