BRACHIOPODA. 37 



Diagnosis. Shell sub-circular ; valves convex, thick. 



Pedicle- valve with an acute and slender beak ; cardinal ax'ea 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 diiference 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 



