VEBTEBBATES. 445 



Bythiacajstthtjs Van Hoenei, St. J. and W. 



PI. XTU, Pig. 1. 



Fin-spines large, exposed portion massive, subelliptical in section, 

 basal line nearly corresponding to the anterior margin, presenting as 

 seen from the side a long elliptical figure, obliquely truncated behind 

 by the low, prominently keeled posterior face, which scarcely equals in 

 hight the greatest transverse diameter of the exposed portion of the 

 spine. Lateral faces very slightly convex and gradually converging to 

 the rounded anterior margin, which is smooth from wear above, and 

 terminated in an obtuse beak; tubercles arranged in more or less regu- 

 lar rows obliquely descending from the anterior edge in a slight forward 

 curvatnre to the basal line, probably multiplying by implantation, the 

 interspaces narrow, faintly depressed and irregularly striato-puuctate, 

 in a mature specimen there being about fifteen such rows diverging 

 from the anterior border and twenty to twenty-five along the inferior 

 border ; the tubercles are large, conical, slightly compressed laterally, 

 or eccentric, with more or less distinct trenchant edges, rising from a 

 broad base, and beautifully sculptured by fine radiating ridges which 

 abruptly terminate below in a faint constriction below which the broad 

 base expands to the body of the spine. The posterior face, as pre- 

 viously remarked, is very low and strongly keeled, the lateral surfaces 

 gently concave and coarsely striato-punctate in the same mauner as the 

 interspaces in the lateral faces; along the crest of the median keel near 

 the apex occur obscure traces of what appear to have been denticles, 

 but this denticulate appearance may be the result of accident. Base 

 constituting more than two-thirds the bulk of the spine, line of inser- 

 tion distinctly marked by the termination of the tuberculose surface, 

 below which it is gently compressed, long elliptical in transverse section, 

 anterior margin rounded and broadly curved interiorly to the obtusely 

 rounded extremity, posterior face deeply excavated by the pulp cavity, 

 which latter apparently terminates at the junction of the posterior face 

 with the line of insertion, exterior surface of the base coarsely and 

 irregularly striato-punctate. 



The remarkable form above noticed is represented by a unique and 

 quite perfect example, "which was discovered by Mr. Van Hoene, to 

 whom we have dedicated the species. We are aware of but a single 

 other allied species, that described by Dr. Leidy tinder the name Aste- 

 racanthus siderius, (Dr. Joseph Leidy, 1873, Ext. Vert. Eauna, p. 313, 

 PI. XXXII, f. 59,) purporting to have been derived from the Lower Car- 

 boniferous deposits near Glasgow, Tennessee, and possibly belonging to 

 the St. Louis division of the group. There seems to be no question as 



