BRracuiopoDa. 831 
are obliquely concave, the outer and anterior angle being much 
elevated and the slope thence to the dental sockets abrupt. The 
crura are attached to the inner margins of these plates, are not 
curved, but their distal extremities are expanded into spoon- 
shaped processes which haye their concave surfaces toward the 
brachial valve. There are no thickened muscular scars as in the 
opposite valve. 
The surface is covered with sharply angular, simple plications, 
most of which begin in the umbonal regions, and the broad fold 
and sinus may bear as many as from eight to twelve of these. 
All the plications are crossed by fine, sharp concentric lines of 
ornamentation, which crenulate the summits of the ridges. 
Shell-substance fibrous, impunctate. 
Type, Cyclorhina nobilis, Hall. 
Distribution. Middle Devonian. 
Terebratuloidea, Waagen. 1883. 
Shells oval or rounded, with strongly plicated valves and a 
high median fold in the brachial, and a corresponding sinus in the 
pedicle-valve. Beak truncated with a terminal round foramen ; 
deltidial plates distinct. 
Teeth strong, not supported by dental plates. Brachial valve 
with a tolerably large triangular hinge-plate, which is triangularly 
Fie. 416. 
Terebratuloidea Davidsoni, Waagen. 
Fia. 415.— View of the exterior. 
Fic. 416.— Interior of the pedicle-valve. 
Fia. 417.— Interior of the brachial valve. CWAAGEN.) 
divided. There is no cardinal process. On both sides of the 
median incision very short curved crura take their origin, and 
proceed for a short distance in a slightly diverging direction 
toward the interior of the shell. There is no median septum in 
the brachial valve. 
Type, Terebratuloidea Davidsoni, Waagen. 
Distribution. Carboniferous — Permian. 
83 
