VERTEBRATES. 237 



or 8 centimetres from the tip before it opened out in the deep, 

 trough-like excavation that extends thence to the proximal ex- 

 tremity. 



As has already been remarked, the specimen described may prove 

 to be specifically identical with that noticed under the head of 

 Utenacanthus de/levus. But this relationship can be satisfactorily 

 determined only from the examination of additional materials, both 

 forms being at the present time known from unique and imperfect 

 examples. The specimen here referred to was obtained by Mr. 

 George B. Harrison, of Bloomington, to whom we are also indebted 

 for other interesting contributions. 



Geological position and locality: St. Louis limestone; Alton, Illi- 

 nois. 



CTENACANTHUS PELLENSIS, St. J. and W. 



PI. XXI, Fig. U 



The collection of Mr. Van Home contains a fragment of a medium- 

 sized spine of a species not hitherto noticed. The specimen pre- 

 serves a length of about 15 millimetres apparently from near the 

 dorsal line, and from the high opening of the pulp cavity and other 

 characters to be noticed further on, it would seem to belong to the 

 section of Ctenacanthi characterized by the great extent of the dorsal 

 line. The fragment indicates a thick, gradually tapering form, obtuse 

 wedge-shaped in cross section, the posterior side deeply excavated 

 by the open trough of the pulp cavity, which is denned by the bev- 

 eled edges of the postero-lateral angles. Lateral surfaces gently 

 convex, somewhat sharply arched into the rounded or subangular 

 anterior edge. The surface ornamentation is very intricate and ele- 

 gant, consisting of numerous delicate longitudinal costze more or 

 less regularly diminishing in size from the anterior edge, more fre- 

 quently bifurcated and deflected on Hearing the postero-lateral angles 

 where they cease at the exterior beveled edge; in front the costae 

 present plain, rounded enameled crests spaced by narrower inter- 

 vening sulci, their lateral edges studded with delicate downward 

 curved transverse carina? or tubercles ; the third rib from the dorsal 

 edge shows more or less distinct undulations, and the fifth rib is 

 surmounted by small stellate tubercles more or less variable in the 

 details of surface sculpture and disposition, their apices directed 

 upward, and which apparently extend over the entire posterior flank 

 of the lateral surfaces. 



