BRACHIOPODA. 
243 
of the internal septal plates. These divisions will be considered more at 
length. 
(A). Galeatiform pentameroids bearing the fold on the brachial valve and the sinus 
on the pedicle-valve. 
Among the Silurian shells of this group there are two types of exterior, one 
plicated {P. fornicatus, P. Areyi, of the Clinton group), and the other smooth 
(P. linguifer, P. ventricosus, of the Wenlock-Niagara). There is, however, little 
• Fig. 173. Fig. 174. 
Fig. 173. Pentamerus (Barrandella) linguifer. Transverse section near the anterior e.xtremity of tlie median sep¬ 
tum in the pedicle-valve; showing the form of the spondylia. (C.) 
Fig. 174. Pentamerus (Barrandella) Barrandii, Billings. A transverse section in front of the termination of the 
median septum of the pedicie-valve; showing the form of the spondylium in both valves, and the 
coalescence of the septa in the (lower) brachial valve. (C.) 
if any difference in internal structure. Davidson has shown* that P. linguifer 
bears a short spondylium in the pedicle-valve, supported only at its posterior 
surface, the free extension being produced forward and upward into the cavity 
of the opposite valve ; and, also, that the crural plates of the brachial valve are 
convergent. The accompanying transverse section of this shell shows that 
these plates are concave on their outer surfaces and are supported by conver¬ 
gent septa uniting as they reach the valve and leaving but a single median line 
of union on its surface. With very slight variation in the degree of conver¬ 
gence of the last-named plates the same structure exists in the P. ventricosus, of 
the Niagara dolomites, a shell whose differences from P. linguifer it may be 
difficult to establish, f In the strongly plicated Clinton species, P. Areyi, the 
internal structure varies in having, so far as known, a well developed spon- 
* Silurian Brachiopoda, pi. xvii, lig. 14 b. 
t This remark refers to the normally smooth shell which served as the type of the species. There is, 
however, a variation closely associated with P. ventricosus, both structurally and in its occurrence, which 
has low plications on fold and sinus, though the lateral slopes are smooth. Figures of this form are given 
on Plate LXXI, 
