The American Naturalist. 
single fold on the larger, or 
the folds in both valves are re- 
placed by a truncation of the 
posterior margin. Arm sup- 
ports reaching not more than 
one-third the length of the shell, Terebratula Llwd. 
(7) Arm supports long {jj). 
{JJ) Both valves convex. Fora- 
men large. Arm-supports 
united by a loop near the 
hinge, Cryptonella Hall. 
{jj) Smaller valve usually 
almost flat, rarely convex. 
Arm-supports united by two 
loops which unite posteri- 
orly to form a short, free 
median plate, Centronella Bill. 
Subgenera : 
Forms with surface ribbed 
or striated, Leptoccelia Hall. 
(m) Beak very short. Shell large. 
Both valves convex, depressed, 
without fold and sinus, Meganteris d'Arch, 
(/) Surface ribbed or striated {k). 
{k) See Leptoccelia above. 
{k) Shell punctate, much elon- 
gated, without fold and sinus, or 
spires, Rettsselaeria Hall. 
{k) Shell punctate. Interior with spires {kk). 
{kk) Beak closely incurved, no area (/). 
(/) Form more or less elon- 
gated. Central ribs narrow. 
Cardinal plate broad, Rhynchospira Hall. 
(/) Form transverse. Under 
the beak of the larger valve 
