BRACHIOPODA. 
131 
obliquely outward, toward the lateral slopes of the pedicle-valve, similar to 
those of Atrypa, though more divergent. 
In 1882, Mr. Davidson introduced* * * § the term Bifida as a generic designation 
for the species above referred to by Schnur as a very close ally of Terehratula 
venusta, viz.; T. lepida, Goldfuss, as quoted by d’Archiac and de Verneuil,! 
the Atrypa lepida, Davidson,^ and the Retzia lepida, Kayser, § a form occurring 
in association with T. venusta in the Devonian of the Eifel, and known also 
in the middle Devonian of Torquay. 
Mr. Davidson has shown, from preparations made by the Rev. Mr. Glass, 
that the spiral coil is loosely arranged, the ribbon making but four or five 
volutions; the apices of the cones are directed obliquely outward toward 
the lateral slopes of the pedicle-valve. The loop 
is described as simple, “like that in Meristina 
[ Whitfieldella], with the exception that it is 
placed nearer to the attachments of the hinge- 
plate, and that, at the point where the two lamellae 
composing the loop join, there is a short bifur¬ 
cation directed upwards” (p. 28). It was this 
alleged bifurcation of the loop that suggested the 
name Bifida, and on the basis of this also that 
Mr. Davidson compared and classified the shell ^avidsox from preparations by glass. 
with the genus Whitfieldia (= Meristina.) The accompanying figure shows 
the structure of this organ as described by him. A careful study of this 
shell 11 has led to a quite different result in regard to the structure of the 
loop. These observations have been made not only from translucent prepara¬ 
tions, but also by consecutive sectioning of opaque specimens, and the 
conclusions verified by frequent repetition of the process. 
The accompanying figures are of sections made from opaque specimens 
cut in a plane vertical to the longitudinal axis of the shell, the series 
* Supplement to British Devonian Brachiopoda, p. 27. 
t Transactions of the Geological Society of London, second ser., vol. vi, p. 386. 1840. 
J British Devonian Brachiopoda, p. 52. 1864. 
§ Zeitschr. der deutschen geolog. Gesellschaft, vol. xxiii, p. 559. 1871. 
I From specimens from Gerolstein in the Eifel, furnished by Di'. E. Holzapfel, of Aachen. 
The brachidium as represented by 
