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 setse 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 onlj^ 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., 184T. (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. eUipHca, 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. (cxl, 88-93). 



Paterula, Barrande. 

 Syn. — Cyclus, Barr. 

 Distr. — P. Bohemica, Barr. Silur, ; Bohemia. 



Family OBOLID^. 



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. 



Obolus, Eichwald, 1829. 



Etym, — Obolus, a small Greek coin. 



Syn. — Ungulites, TJngula, Pander, 1830. Aulonotreta, 

 Kutorga, 1848. 



Distr. — Fossil, 8 sp. Lower and Upper Silurian ; Sweden, 

 Russia, England, United States. 0. Davidsoni, Salter (cxl, 

 94, 95\ 



Shell orbicular, calcareo-corneous, depressed, subequivalve, 

 smooth ; hinge-margin thickened inside, and slightly grooved 



