The Genera of Brachiopoda. 991 
slight. Both valves 
with an area and foramen. No 
deltidium. Area not striated. 
Hinge line usually less than 
breath of shell. Sinus and fold 
usually absent, rarely prominent, OrtJiis Dalm. 
Subgenera. 
Shell two-lobed through the pres- 
ence of a sinus in each valve, Bilobites Lin. 
Very convex forms, with hinge 
line often exceeding the breadth 
of the shell, and a prominent 
sinus and fold, Platystrophia King. 
{tf) Shell with a wide area, foramen, 
and pseudodeltidium in eich 
valve, Orthisina d'Orb. 
{ti) Larger valve pyramidal. Other- 
wise Orthis-like, Skenidium Hall. 
(/) Shell nearly spherical, punctate 
thin hinge line very short. Area 
small. Beak recurved, Syntrielasma M. & W. 
(^) Shell triangular, varying towards 
circular. Sinus and fold usually 
prominent. Hinge line often ex- 
ceeding the breadth of the shell. 
Beak usually more prominent than 
in Orthis. Area in both valves, 
that in the ventral larger, and stri- 
ated with horizontal and vertical 
lines. Spires present, Spirifcr Sow. 
Subgenera. 
Forms with punctate shell {v). 
{v) Surface finely spinose. Fora- 
men covered by a deltidium. 
Hinge line sometimes curved, Spiriferina d'Orb. 
{v) Hinge line long and straight. 
Larger valve with a broad area, 
