252 PALEONTOLOGY OF ILLINOIS. 



where, as already noted, they appear to have the oblique arrange- 

 ment common to the genus to which the species represented by 

 the above specimen is provisionally referred, and of which it is, at 

 the present time, the sole representative from the Carboniferous 

 formations of the Upper Mississippi region. Principal Dawson has 

 noticed representatives of the genus from the Carboniferous of Nova 

 Scotia, and Dr. Newberry has described some interesting species 

 from the Lower Carboniferous of Ohio. 



Geological position and locality : Keokuk limestone ; Keokuk, Iowa. 



GENUS PHYSOTSTEMUS, Agassiz. 

 PHYSONEMUS FALCATUS, St. J. and W. 



PI. XXIV. Fie. 6. 



Dorsal spine of small size, comparatively narrow, gradually taper- 

 ing, and very strongly arched along the posterior face, the tip reach- 

 ing a point nearly vertical to the inferior extremity. Transverse 

 section wedge-shaped, anterior edge making an acutely rounded 

 angle, expanding below into the inferior shoulder, which is sharply 

 constricted at the dorsal line to the plain base. Lateral surfaces 

 slightly convex, the bony surface showing an impressed line parallel 

 with the posterior edge as usually observed under similar conditions 

 of surface exfoliation. 



The description refers to a unique example belonging to the col- 

 lection of Washington University. It represents nearly the en&re 

 outline, but as so frequently occurs with the small specimens of the 

 genus, the posterior face is worn away, the pulp cavity making a 

 groove throughout its exposed extent. The exposed portion of the 

 spine, also, is denuded of the original longitudinal costse and tuber- 

 culation, of which not a vestige remains from which to infer the 

 character of the superficial ornamentation. The strong curvature of 

 the spine recalls the forms to which the names Physonemus parvulus, 

 and Ph. Cestriensis, derived respectively from the Keokuk and Ches- 

 ter formations, have been given. It is, however, a much more slender 

 form, not attaining near the breadth of the above named spines. It 

 is, indeed, possible that it prove to be a young individual of the 

 species Ph. Altonensis, St. J. and W., of the same formation, though 

 the latter identity seems highly improbable. 



Geological position and locality: St. Louis limestone ; St. Louis, 

 Missouri. 



