240 . PALAEONTOLOGY OF ILLINOIS. 



the costcie ridges or tubercles, which are very appreciably more regu- 

 lar and delicate than is the case in the St. Louis form. The 

 magnificent exampj^ of the latter spine belonging to the Shumard 

 collection of Washington University, affords unusually satisfactory 

 details relating to the ornamentation of the exposed parts, also the 

 changes in the superficial characters toward the distal extremity 

 attributable to wear, the careful examination of which has led to 

 the belief that- the spine under consideration belonged to a distinct 

 species. 

 Geological position and locality: Chester limestone; Chester, Illinois. 



CTENACANTHUS BUTTERSI, St. J. and W. 



PL XXII, Fig. 2. 



Fin-spine of medium size, stout, slightly arched along the anterior 

 edge, gradually tapering, subtriangular in cross section ; the imbedded 

 p:oximal extremity much more rapidly tapers, gently depressed 

 below the anterior shoulder, relatively more expanded and triangular 

 in section, the dorsal line making a broad curve, the concommitant 

 plain space reaching high up toward the distal extremity even be- 

 yond the point of inclosure of the pulp cavity. The pulp cavity is 

 inclosed posteriorly for only a comparatively short distance from the 

 apex, where it opens out, forming thence a deep angular trench in 

 posterior face reaching to the proximal extremity; its section ap- 

 proximates that of the external walls, its dimensions and position 

 being shown in the section diagrams introduced with the illustrations 

 of the type specimen. The posterior face appears to be slightly 

 convex transversely and sharply rounded at the lateral angles, the 

 specimens not showing denticles, if indeed such exist. The lateral 

 surfaces are gently convex and more or less obtusely rounded along 

 the anterior edge which bears a single longitudinal rib ; the exposed 

 surface is regularly and somewhat strongly ribbed, the costae of 

 moderate prominence and convexity, with narrow intercostal furrows, 

 and chiefly bifurcating near the postero-lateral angles, where also 

 short implanted costae occur somewhat suddenly deflected to the 

 narrow, plain belt bordering either posterior margin, along which 

 they abruptly cease. The anterior ridges are interrupted by strong, 

 crowded, transverse tubercles, which have a slight oblique direction 

 backward and upward, the crest culminating above with gentler in- 

 ferior slope, and delicately sculptured by radiating carinae; the 



