VERTEBEATES. 449 



more or less profound excavatioa along the postero-iuferior borders of 

 the spine. Basal portion compressed beneath the antero-iuferior shoul- 

 der, often deeply notched in front, and more or less produced anteriorly, 

 terminating in an acute or rounded extremity ; line of insertion marked 

 by the termination of the costation of the exposed lateral faces. Attain 

 large size. 



A striking feature observed in a large collection of these ichthyodoru- 

 lites, is the denuded condition of the external surface by which the 

 tubercnlation has been removed, and in most instances every vestige of 

 the costation has also disappeared, leaving a perfectly smooth surface, 

 in which latter condition they would be most puzzling objects but for 

 the fortunate possession of specimens exhibiting various conditions of 

 preservation. As often occurs in spines whose surfaces are studded 

 with prominent, isolated tubercles, the superficial characteristics are 

 seldom preserved in other than mutilated condition : but it is notable 

 amongst these Physonemi that very few examples show all the distinc- 

 tive features of the genus, or more than the general outline, and under 

 these circumstances a perfectly trustworthy definition of their specific 

 characteristics cannot, in many cases, be given, or at least carried 

 beyond the particulars of form and proportion. 



There are also other characters which pertain to the development of 

 the individual, as indicated in apparently old and young specimens of 

 the same species, by which the outline of extreme examples offers con- 

 siderable contrast, as noticed in connection with the description of the 

 Kiuderhook form, Ph. proelivus. But probably the origiu of most of 

 these eccentricities in shape is directly traceable to the effects of abra- 

 sion, by which the comparatively thin posterior walls of the spine have 

 been destroyed, opening to view the entire extent of the internal cavity, 

 which in the majority of specimens before us forms a more or less deep 

 channel in the posterior face, gradually becoming obsolete on approach- 

 ing either attenuated extremity. So great are the modifications in out- 

 line produced by the accidental removal of a greater or less extent of 

 the posterior portion, that it is often difficult to appreciate the specific 

 identity of individuals exhibiting different degrees of abrasion, as 

 instanced in the examples of Ph. Altonensis of the St. Louis formation. 



From the figures of Ph. arcuatus, McCoy, it is apparent Prof. McCoy 

 possessed a nearly perfect specimen. The original of Ph. gigas, N. and 

 W., of the Upper Burlington limestone, though quite perfect as to the 

 completeness of the body, the exposed surface does not exhibit the 

 tuberculose ornamentation of the longitudinal costaj which formed a 

 beautiful feature of its ornamentation, and which is shown in specimens 

 subsequently obtained by Mr. Springee and one of the writers. The 

 entire surface was beautifully tuberculated, the tubercles being closely 



