BRACHIOPODA. 141 



Of DisciNOLEPis little is known except its external conformation, while ScHizo- 

 CRANiA expresses the extreme result of this tendency to marginal development 

 in the aperture, an effect which comes by the way of Trematis. 



Two months after the publication of the term Trematis by Sharpe, D'Orbigny 

 proposed the name Orbicella for similar fossils having a supposed punctated 

 shell-structure and a convex pedicle-valve ; but no typical example was cited 

 by him until, in his "Prodrome de paleontologie stratigraphique,"* nine species 

 were referred to his genus, the first of which is the Orbicula Buchi ofVERNEUiL.f 

 It appears evident that D'Orbigny's comprehension of his genus was equivalent 

 to that of iSharpe for Trematis, as in his list both Orbicula terminalis, Emmons, 

 and 0. ? punctata, Sowerby, are cited. It is necessary, however, to take Orbicula 

 Buchi as the type of the Orbicella, and it does not appear from Verneuil's 

 description that this species is congeneric with Trematis terminalis. No mention 

 is made of a punctated external layer, though this may have been accidentally 

 absent in Verneuil's specimens ; the fissure is described as lanceolate and not 

 extending to the boi'der.j: 



It has been observed elsewhere that this Orbicula Buchi is the species taken 

 by Pander in 1861 as the type of his genus Keyserltngia (see page 117), and 

 that by a strict construction of the rules of precedence, Orbicella must stand 

 in place of Pander's term. Thus while D'Orbigny's Orbicella, under the 

 author's conception of the group, is synonymous with Trematis, and must be 

 abandoned in this connection, it is rehabilitated as a genus by Pander's invest- 

 igations. 



The genus Trematis appears to be largely confined to American Silurian 

 faunas. The published evidence in regard to the character of the pedicle- 

 aperture in all the British species referred to this genus in the lists accompany- 

 ing Davidson's General Summary (T. Siluriana, Davidson, T. punctata, Sowerby, 



* 1849, p. 20. 



t.Geol. Russie d'Europe et des mont. de I'Dural, p. 228, pi. xix, fig. 1. 1845. 



X It may be noticed that Mr. Davidson's elegant figure of Trematis 'punctata, as well as the description 

 of the same species (British Silurian Brachiopoda, p. 69, pi. vi, fig. 9fi), gives the pedicle-fissure the same 

 character ; probably an error arising from imperfect preservation of the specimens, but in case the charac- 

 ter of the pedicle-aperture has been correctly represented, the species can not be congeneric with Trematis 

 terminalis. 



