BRACHIOPODA. 243 



of the internal septal plates. These divisions will ])e considered more at 

 length. 



(A). Galeatiform peniameroids 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). Tiiere is, however, little 



Fig. 173. Fio. nt. 



Fig. 173. Pentamerus { Barranddla) linguifer. Transverse section near the anterior extremitj- nf tlio median sep- 

 tum in tlie pedicle-valve; showing the form of the spondylia. Ic.) 



Fig. 174. Pentamerus {Barrandetla) JSarrandii, Billings- A transvers"? section in front of the termination of the 

 median septum of the pedicle-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.! 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, fig. 14 6. 



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. 



