The American Naturalist. [November 
a narrow triangular fissure partly- 
closed by a deltidium, and a broad 
median sinus. Smaller valve 
without area, Syringothyris Winch. 
{v) Punctate shells which closely 
resemble Cyrtia, Cyrtina Dav. 
Forms with impunctate shell {vv). 
{vv) Shell gibbous. Surface 
smooth or finely striated. Ven- 
tral valve very convex, with a 
prominent incurved beak. 
Dorsal flat, or much less con- 
vex, Martitiia McCoy. 
{vv) Surface ribbed. Larger 
valve high pyramidal, with a 
large, flat, triangular area, and 
a median fissure closed by a 
pseudodeltidium. Smaller 
valve convex, Cyrtia Dalm. 
