BRACHIOPODA. 327 



is to be observed that specific distinctions in the group are established with 

 great difficulty on account of the persistence of the type of structure without 

 essential modification. 

 Examples : 



P. semireticulatus, Martin. P. Wortheni, Hall. 



P. arcuatus, Hall. P. magnus, Meek and Worthen. 



P. costatus, Sowerby. P. nanus, Meek and Worthen. 



P. Burhngtonmsis, Hall. P. Lasallensis, Worthen. 



P. Altonensis, Norwood and Pratten. P. temiicostatus, Hall. 



P. Chesterensis, Worthen. 



IV. Spinosi, de Verneuil. Surface strongly tuberculose or spinose ; not 

 reticulated. 



Examples : 



P. Nebrascensis, Owen. P. scabriculus, Martin. 



P. asper, McChesney. P. marginicinctus, Prout, 



V. Fimbriati, de Koninck. Surface without radiate strioa or ribs; covered 

 with concentric ridges or plications, bearing rows of small, thickly set spines. 



Examples : 



P. puncfatus, Martin. P. alternatus, Norwood and Pratten. 



P. vittaius, Hall. P. symmetricus, McChesney. 



P. biseriatus, Hall. P. Rogersi, Norwood and Pratten. 



VI. Horridi, de Verneuil. Surface without radiating or concentric plications ; 

 pedicle-valve with a deep sinus. It is doubtful if any member of this group is 

 known from American faunas. Among European species are : 



P. horridus, Sowerby. P. opuntia, Waagen. 



P. Geinitzianus, de Koninck. 



VII. Mesolobi, de Koninck. Surface without radiating plications, except a 

 prominent median rib. Unknown in this country. 



Examples in the European Devonian are : 



P. mesolobus, Phillips. P. Christiani, de Koninck. 



