BRACHIOPODA. 157 



generally, appearing in the earliest known representative, P. Trentonmsis, 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. hellula of the Lower Devonian of the Eureka 

 District, P. lepis of the Corniferous limestone, P. Hamiltonia of the Hamilton, and 

 an undetermined species in the Waverly, and also P. anliqua 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. Imguloides 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 



