VERTEBRATES. 455 



and yet in every other respect their resemblance is so intimate as to 

 leave no doubt as to their specific identity. The breaking away and 

 erosion of the posterior borders results in reducing the entire posterior 

 aspect to a curved line corresponding more or less nearly with the ante- 

 rior limit of the internal cavity, the lateral walls becoming the bounda- 

 ries of the more or less deep channel which it forms in the posterior 

 side. In the spines under consideration, by this means the diameter of 

 the body is reduced to less than one-third its entire breadth, besides 

 greatly reducing its vertical elevation, so as -to give the basal portion 

 disproportionately large dimensions, though the anteriorly produced 

 portion suffers equally from abrasion, being diminished horizontally 

 and vertically to the extent of the thin lateral walls protecting the infe- 

 rior prolongation of tne pulp cavity ; hence, it will be observed, the 

 process of abrasion is chiefly confined to the posterior and inferior bor- 

 ders, and in the successive stages of its progress, the region in the 

 vicinity of the anteroinferior shoulder assumes successively a relatively 

 more and more exaggerated predominance. 



Amongst some small spines obtained from the same formation at 

 Pella, Iowa, there are several fragments of Physonemi. of which the 

 only appreciable distinction, aside from their relatively diminutive size, 

 consists in the truncation and broadly rounded extremity of the antero- 

 inferior shoulder and the consequent reduction of the notch in front, the 

 sharp superior edge of the base terminating at the edge of the shoulder. 

 The condition of these latter spines recalls that of the Kinderhook form 

 to which we have referred under the name Ph. proclivus, and which 

 holds about the same relation in this particular to Ph. depressus of the 

 same deposits, as do the present examples to the large specimens 

 described above, the origin of which may be partly due to abrasion and 

 partly to the development of the spine. 



Position and locality : In the upper beds of the St. Louis limestone ; 

 Alton, Illinois, and Pella, Iowa. 



Physonemtjs Chestekensis, St. J. and W. 



PL XIX, Fig. 4. 



Dorsal spine represented by a small and imperfect specimen, in which 

 the external ornamentation is entirely obliterated and the posterior 

 portion worn away so as to expose the external cavity, which forms a 

 moderate depression in the present aspect of the posterior face. The 

 basal portion is also broken away, and the anteroinferior shoulder worn 

 down so as to appear as a moderately laterally expanded, depressed 

 prominence. Outline strongly curved, transverse section long-elliptical, 



