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 Sharpe 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 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 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 Avith 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 l’Oural, p. 228, pi. xix, fig. 1. 1845. 
} 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. 9 a), 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. 
