BRACHIOPODA. 
157 
generally, appearing in the earliest known representative, P. Trentonensis, and 
enduring until the genus disappears in the Lower Carboniferous. Among the 
forms which follow this type of exterior, it is often difficult to point out specific 
differences, variations in internal features accompanying the slight external 
changes in outline or convexity being as a rule difficult to establish. Thus the 
forms, P. Trentonensis from the Trenton limestone, P. Cincinnatensis from the 
Hudson group, P. squamiformis of the Niagara and Clinton groups, P. ovata 
of the Lower Helderberg, P. bellula of the Lower Devonian of the Eureka 
District, P. lepis of the Corniferous limestone, P. Hamiltonice of the Hamilton, and 
an undetermined species in the Waverly, and also P. antiqua of the Beyrichien- 
kalk of the Island of Gotland, present differences of slight value in any respect, 
but each occurs at a distinct horizon and serves to characterize the faunas of 
which they are respectively members. 
A slight variation in external outline without essential change in interior 
characters is seen in the elliptical species, P. ovalis of the Niagara and in P. 
implicata of the Wenlock; and it is probable that these American and European 
forms are identical. A more extreme variation appears in the species, P. termi- 
nalis of the Oriskany sandstone, P. calceola of the Corniferous, P. linguloides and 
P. oblata of the Hamilton, where the apex is terminal and produced, the con¬ 
centric striae on the posterior limb of the shell making a sort of cardinal area. 
This extended umbo is solid and it remains to be ascertained whether it occurs 
on both valves alike. 
In regard to the nomenclature of the two valves in Pholidops, the terms 
pedicle and brachial are inapplicable here as in Crania, on account of the absence 
of any pedicle-aperture in the mature shell. The conventional designations 
ventral and dorsal, which are virtually misnomers in their application to the 
brachiopoda, may serve to indicate a means of comparison with the attached 
(“ ventral ”) and free (“ dorsal ”) valves of Crania. With the latter, that valve 
in Pholidops is homologous which bears the median pair of narrow, elongate 
scars, traversing the anterior apex of the muscular callosity; these scars 
representing the distal anteriors found in the free valve of Crania (probably 
the anterior insertion of the brachial muscles), and shown very distinctly by 
