430 PALAEONTOLOGY OF ILLINOIS. 



and more circumscribed in the posterior ridges, where they are more 

 widely spaced. Pulp cavity elliptical iu section, about one third the 

 lateral diameter of the body of the spiue, and apparently extending 

 to the line of insertion before appearing in the posterior face. Base of 

 moderate length, strong, moderately excavated posteriorly, line of inser- 

 tion forming an angle of about 45° with the posterior inclination of the 

 spine. 



The present form is recognized from a couple of fragments of spines 

 in the collection of Mr. Van Hoene, one of which shows about three 

 inches of the inferior extremity of a mutilated specimen, the smaller 

 fragments exhibiting a section from the middle region of the spine in 

 a beautifully perfect state of preservation. Neither of these frag- 

 mentary examples preserves any trace of denticles along the postero- 

 lateral angles, though such probably existed, possibly confined to the 

 upper extremity of the spine. 



In outline it bears intimate resemblance to G. gracillimus, N. and W. 

 of the same horizon, its peculiar and generally distributed tuberculatum 

 constituting its chief distinguishing features. The latter, however, 

 show a striking likeness to the Upper Burlington spines which we have 

 noticed uuder the name G. gradocostus, from which the present form 

 differs iu the relatively greater size and more elaborately sculptured 

 tubercles of the anterior costse, and the flattened, faintly keeled poste- 

 rior face. 



Position aud locality: Iu the upper beds of the St. Louis limestone; 

 Alton, Illinois. 



Ctenacanthus ptjgiunculus, St. J. and W. 



PI. XXI, Fig. 9. 



Fin-spine long, nearly straight or but gently curved posteriorly, 

 gradually tapering to the acute apex, suboval in transverse section. 

 Lateral faces regularly and broadly rounded into the anterior margin, 

 the longitudinal costte, of which there are nine or ten in either side, of 

 uniform size and spaced by about their own diameter in front, but 

 diminishing in size posteriorly and more closely approximated, decussa- 

 ted, or those in front bearing more prominent, oblique tubercles, sepa- 

 rated by vertical spaces equal to two or three times their diameter, the 

 posterior ones less widely separated or more numerous, forming sharp, 

 oblique ridges abruptly terminated above ; the tubercles are apparently 

 smooth and polished, and there appears to be much regularity in the 

 direction of their obliquity, or ascending from the anterior to the poste- 

 rior side, though probably subject to variation iu the latter respects. 

 Posterior face equal to the greatest lateral diameter, or nearly so, of the 



