236 PALEONTOLOGY OF ILLINOIS. 



CTENACANTHUS HARRISONI, St. J. and W. 



PL XXIII, Fig. 1. 



The collection from the St. Louis formation contain an example 

 of a large spine which is apparently intimately allied to the last 

 preceding species, Ctenacanthus dejiexus, but which presents certain 

 well-marked peculiarities at variance with the distinctive features of 

 that species. These differences do not appear to be such as might 

 be attributable to age, and on the whole we are inclined to regard 

 them as being of specific importance. 



The specimen under consideration shows the greater part of a 

 spine which probably attained a length of between 20 and 25 centi- 

 metres, and it apparently belonged to the broad-based type of which 

 the preceding species constitutes a typical representative. Present- 

 ing about the same degree of curvature along the dorsal edge, it 

 more gradually tapers, and the transverse section is less compressed 

 laterally, approaching nearer a triangular outline than is the case 

 in the former species, which, however, it resembles in the solid tip 

 and comparatively limited extent of the closed pulp cavity. The 

 specimen is much worn, but not sufficient to obliterate the limits of 

 the dorsal line, the position of which is indicated by a symmetrical 

 undulation in the abraded surface extending at a sharp angle back- 

 ward and upward from the inferior dorsal angle ; the posterior edges 

 of the lateral walls are also evidently rounded by abrasion, so that 

 it is difficult to make a satisfactory restoration of the outline of 

 these edges that will accord with the known form of the correspond- 

 ing portion of the species with which the present spine is especially 

 compared. Along the dorsal edge, which is broadly rounded into 

 the moderately convex sides, several rows of densely tuberculated 

 costae are still retained in a very perfect state of preservation, 

 showing the transversely compressed tubercles with abrupt declivity 

 above and steep slope below, delicately sculptured by vertical or 

 radiating carinac, the tubercles spaced by about their own diameter 

 vertically and those of one row separated from the contiguous row 

 by a mere line ; over the remainder of the surface the costas are 

 obsolete. The posterior face is quite convex, with a low, rounded 

 median angulation, and, in the specimen, sharply rounded into the 

 lateral surfaces ; there remains not a vestige of denticles along the 

 postero -lateral angles. The posterior wall of the pulp cavity has 

 been broken away, it originally extended a distance of at least 7 



