1889.] 
The Genera of Brachiopoda. 
ea. No 
prominent. JNi 
fold or sinus, Pentavierus Sow. 
Forms which have a narrow 
area in the ventral valve, 
and a fold and sinus, Petitamerella Hall. 
Exceedingly short and gibbous 
forms, having an area in both 
valves, Gypidula Hall. 
Forms which have the ventral 
valve smaller, and less con- 
vex, than the dorsal, Anastrophia Hall. 
See also Amphigenia above, and Spiririferina below. ^ 
(A) Hinge line straight if). 
if) Shell concavo-convex (rr). 
{rr) Surface spinose, especially along the hinge {s). 
{s) Large valve very convex, or ab- 
ruptly bent " knee form." Beak 
large, abruptly incurved. Smal- 
ler valve more or less concave. 
Area linear or absent. Hinge 
toothless, Productus Sow. 
Subgenera. 
Forms with a low area on both 
valves, and hinge teeth, Prodnctella Hall. 
Forms with a high area on the 
ventral valve. Hinge toothless, Aulosteges Helm. 
[s) Shell broadly transverse. Beak 
not prominent. Low area on 
both valves. Spines along hinge, Chonetes Fischer. 
{rr) Surface not spinose {ss). 
{si) Shell semicircular or trans- 
verse. Beak not prominent. 
Area of the larger valve low, 
with a triangular opening beneath 
the beak. No deltidium. Area 
of smaller valve linear or absent, Tropidoleptus Hall. 
