BRACHIOPODA. 141 



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

 cRvxiA expresses the extreme resnlt of this tendency to marginal development 

 in the aj^erture, an effect wliich comes by the way of Trumatis. 



Two months after the pnblication of the term Tri:m.\tis by Sharpe, D'Orbigny 

 proposed the name Orbiteij.a 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 stratigraphiqne,"* nine sjjecies 

 were referred to his genus, tlie first of which is the Orbicula Buchi of Verneuil.t 

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

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

 and 0. ? punctata, Sowerby, are cited. It is necessar\-, however, to take Orbicula 

 Buchi as the type of the Orbkella, 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 Yerneuil's specimens ; the fissure is described as lanceolate and not 

 extending to the border.^ 



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

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

 that by a strict construction of tlie rules of precedence, Orhicella 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 TREMATrs, 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- 

 apertux-e in all the British species referred to this genus in the lists accompan}-- 

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



* 1849, p. 2U. 



t Geol. Riissie d'Europe et des raont. de I'Oural, p. 22S, pi. xix, fig-. 1. 1845. 



t It may be noticed that Mr. D.wid.son'.s elegant figure of Tremafii punctata, as well as the desci-iption 

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

 character ; probably an eri-oi- aiising from imiierft'ct preser\ation 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. 



