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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. arcuatus, Hall. 
P. costatus, Sowerby. 
P. Burlingtonensis, Hall. 
P. Wortheni, Hall. 
P. magnus, Meek and Worthen. 
P. nanus, Meek and Worthen. 
P. Lasallensis, Worthen. 
P. Altonensis, Norwood and Pratten. P. tenuicostatus, 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 striae or ribs; covered 
with concentric ridges or plications, bearing rows of small, thickly set spines. 
Examples: 
P. punctatus, Martin. P. alternatus, Norwood and Pratten. 
P. vittatus, 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. 
