338 OBOLID. 
valves opposed to each other. Some of these are probably 
inserted in the pedicel. The oral cirri are extremely tender and 
flexible, contrasting with the stiff and brittle sete of the mantle, 
which are themselves setose like the bristles of certain annelides 
(e. g., the sea-mouse, Aphrodite). The relation of the animal to 
the perforate and imperforate valves is shown to be the same as 
in Terebratula by the labial fringe; but the only process which 
can possibly have afforded support to the oral arms is developed 
from the centre of the ventral valve, as in Crania. Baron 
Ryckholt has represented a Devonian fossil from Belgium, with 
a fringed border; but if this shell is the Crania obsoleta, of 
Goldfuss, the fringe must belong to the shell, and not to the 
mantle. 
In some species the valves are equally convex, and the 
foramen occupies the end of a narrow groove. 
ORBICULOIDEA, d’Orb., 1847. (Schizotreta, Kutorga, 1848.) 
Perforation at the posterior, instead of the anterior, end of the 
internal furrow, which last is impressed from the outside, instead 
of from the inside, as in Discina. D. elliptica, Kutorga. 
DIScINISCA, Dall, 1871. Lower valve more or less flattened, 
concave or compressed, upper valve more convex ; apices of both 
subcentral or subposterior ; lower valve with a small septum, as 
in Discina, behind which is a disk or area impressed from the 
outside, and traversed by a longitudinal fissure in the median 
line of the valve; shell more or less horny in texture, minutely 
tubulous. Silurian—Recent. D. lamellosa, Brod. (exl, 88—93). 
PATERULA, Barrande. 
Syn.—Cyclus, Barr. 
Distr.—P. Bohemica, Barr. Silur.; Bohemia. 
Famity OBOLIDA. 
Shell somewhat inequivalve, rounded or oblique, calcareo- 
corneous ; hinge-margin thickened, and grooved for the passage 
of the peduncle; posterior adductor scars more or less distant 
from the median line. 
Oxsotus, Eichwald, 1829. 
Etym.—Obolus, a small Greek coin. 
Syn.— Ungulites, Ungula, Pander, 1830. Aulonotreta, 
Kutorga, 1848. 
Distr.—Fossil, 8 sp. Lower and Upper Silurian; Sweden, 
Russia, England, United States. O. Davidsoni, Salter (exl, 
94,95). 
Shell orbicular, caleareo-corneous, depressed, subequivalve, 
smooth; hinge-margin thickened inside, and slightly grooved 
