VERTEBRATES. 451 



Enniskxllen, it appears that Cladacanthus paradoxus, Ag., is identical 

 with, and, as I understand it, founded upon the Onchus hamatus. Since 

 it appears very uncertain what relation Physonemus subteres, Ag., holds 

 to the spines represented by Ph. arcuatus, McCoy, it may result in the 

 transfer of all the species heretofore described under Physonemus, except 

 Ph. subteres, the original form recognized by Professor Agassiz, to Clad- 

 acanthus, Ag. 



PHYSONEMI OF THE KIKpERHOOK. 



Ph. peoclivus, Ph. depressus, Ph. carinatus. 



PI. XVIII, Fig. 1, 2, 3, 4, 5. 



The following forms are determined from worn specimens, none of 

 which retain a complete transverse section of the exposed portion of the 

 spine, the posterior walls, which were doubtless very thin, having been 

 destroyed, thus exposing to view the internal cavity which forms in the 

 present condition of the spines a more or less deep channel in the pos- 

 terior face in front of which the solid anterior body extends to the ante- 

 rior edge. In shape and size the specimens exhibit much individual 

 variation, and which may possibly be attributable to the greater or less 

 abrasion to which the individual specimens have been exposed, and by 

 which their outline has been more or less modified. Under these cir- 

 cumstances it is exceedingly difficult to determine the relative import- 

 ance of certain characters, as the extreme erect and depressed position 

 of the exposed body, the relative prominence of the anteroinferior 

 shoulder, and the anteriorly produced basal portion, though they may 

 prove to be dependent upon age, or indicative of developmental origin. 



Taking into consideration the apparent characters, in the absence of 

 those which would afford a knowledge of the superficial ornamentation, 

 but of which latter none of our specimens reveal the faintest trace, two, 

 perhaps three, more or less distinctly marked forms occur, to which we 

 have applied the distinctive terms Ph. proclivns, Ph. depressus and Ph. 

 carinatus, in allusion to the predominant feature by which they are 

 respectively distinguished in the imperfect state of preservation of the 

 spines before us. 



Physonemus proclivus. 



PI. XVIII, Fig. 1,2. 



Authentic examples of the present form have thus far been obtained 

 only from the lower fish-bed of the Kinderhook, and of which there are 



