BRACHIOPODA. 
247 
ian of the Hartz Mountains,* resembling P. galeatus in contour though some¬ 
what shorter and more sharply costated. If the significance of this variability 
be restricted to a specific or even more subordinate value, we shall have a group 
of Silurian shells essentially equivalent to Barrandella, comprising such species 
as P. galeatus, P. Roemeri, nom. propos. (= P. gakatus, Roerner, op. cit.), P. nucleus. 
Hall, P. unipUcatus, Nettelroth, P. Sieberi, von Buch. All these shells are without 
evidence of cardinal area or deltidial plates, have the plication of the surface 
more strongly developed upon fold and sinus, and agree in the internal structure 
of the pedicle-valve. As the designation Sieberella has been brought into use 
for one of these species, we may take the liberty of broadening its significance 
by basing it upon more stable characters than those selected by its author, and 
applying the term to all shells of this type of structure, taking no account of 
the specific variability in the internal structure of the brachial valve. 
Typical Swedish and English Silurian specimens of Pentamerus {Sieberella) 
galeatus possess a peculiar surface sculpture consisting of very fine, irregularly 
anastomosing concentric lines, and, in rare instances, a similar character is pre¬ 
served in the Lower Helderberg specimens of the same species. In regard to 
the various shells from the Devonian that are referred to P. galeatus by the 
European palaeontologists, it may be suggested that they are less likely to 
represent this specific type than to indicate the presence, in those faunas, of 
shells referable to the Devonian genus Gypidula. 
This group Gypidula, Hall, 1867, includes those galeatiform shells of the 
second division which have a well-defined, true, cross-striated cardinal area, 
and narrow, but erect or convex, incip¬ 
ient deltidial plates. On the interior 
the teeth are unusually strong, the 
septum of the pedicle-valve very short, 
the spondylium being free for most 
of its length. In the opposite valve 
the dental sockets are distinct, the 
crural plates expanded nearly horizontally, being divided at their beginning 
Fig. 177. 
Pentamerus (Gypidula) comis, Oweu. 
A transverse section in front of the short median septum of 
the pedicle-valve; showing the form of the spondylia. 
(c.) 
Q- <; p I 
* Kaysbr, Abhandl. zur geol. Specialkarte von Preussen, etc., Band 2, 
figs. 1-9, 13. 1878. 
heft 4, p. 156, pi. xxvii. 
