BRACHIOPODS. 71 



Shell attaining a large size, semi-oval, the breadth being 

 nearly always greater than the length, but varying from about 

 equal to the latter to the proportions of near nine to seven; 

 hinge line as long at the breadth of the valves at any point 

 farther forward, or somewhat longer ; lateral extremities rec- 

 tangular, sometimes compressed and moderately deflected ; 

 lateral margins straight, a little convex, or slightly sinuous 

 posteriorly, and rounding forward to the front, which is semi- 

 circular in outline, or sometimes so prominent and narrowly 

 rounded in the middle as to impart a sub-trigonal form to the 

 general outline of the valves. Dorsal valve flattened in the 

 umbonal and cardinal regions, and gently or more or less 

 strongly concave in the central and anterior portions, and 

 curved upward around the anterior and lateral margins ; beak 

 small, but projecting slightly beyond the edge of the area, 

 which is very narrow, or sub-linear, and directed nearly back- 

 ward. Interior with cardinal process strong directed obliquely 

 forward, with its two divisions distinctly diverging and flat- 

 tened and longitudinally striated on their posterior faces ; 

 sockets for the reception of the teeth of the other valve 

 rather well defined ; socket ridges very small, and uniting 

 behind the cardinal process to form a kind of false deltidium ; 

 muscular scars comparatively small, but deeply impressed near 

 the cardinal process on each side of a small, short, mesial 

 ridge, and nearly surrounded by a low obtuse ridge formed by 

 the thickening of the adjacent internal surface of the valve; 

 anterior and lateral margins more or less thickened and genicu- 

 lated within ( especially in adult shells ), the thickened zone 

 being trans verselyfurro wed and sometimes granular, while out- 

 side of it the immediate edge of the valve is suddenly flattened, 

 and minutely striated and granulated. 



Ventral valve a little convex at the umbo, but generally 

 much compressed over the whole visceral region, in the adult 

 (which included the whole surface of the young and half-grown 

 shell), but becoming more convex (sometimes strongly so) ante- 

 riorly or anterio-centrally and laterally, and thence more or 

 less curved up to the anterior and lateral margins ; area of 



