426 PALAEONTOLOGY OF ILLINOIS. 



the specimens examined presenting evidences of wear; intercostal spaces 

 generllay less tban the diameter of the costas, forming deep, narrow 

 grooves. Posterior face forming an obtuse angle along the median 

 ridge, either side of which is faintly channeled and coarsely striated 

 longitudinally ; postero lateral angles angularly rounded but well- 

 defined, and apparently destitute of denticles throughout the greater 

 length of the spine. Pulp cavity relatively large. Base unknown. 



A fragment of a spine from Quincy, indicating a specimen at least 

 ten inches in length, PI. XV, fig. 2, and a few very imperfect individuals 

 from Louisa county, constitute the meagre material from which the 

 above description is derived. The Quincy specimen, however, shows 

 about two inches of the middle or lower portion of a finely preserved 

 spine, which plainly exhibits the specific peculiarities of the form 

 as compared with others from the same and other horizons. In the 

 style of its tuberculose ornamentation it recalls C. Mayi, N. and W., 

 from the same position, its long and gradually tapering outline and 

 prominent posterior keel, as also the narrower costse, present characters 

 which readily distinguish it from that species. In form and general 

 proportions it also intimately resembles C. Burlingtonensis; but the 

 prominently keeled posterior face and the general tuberculcation of the 

 costse, offer marked contrast with that form. 



Position and locality : In the fish bed of the Upper Burlington lime- 

 stone ; Quincy, 111., Louisa Co., la. 



Ctenacanthus Burlingtonensis, St. J. and TT. 



PI. XV, Fig. 6, 7. 



Fin-spines long, slender, slightly curved aud very gradually tapering, 

 laterally compressed, lenticular in transverse section, truncated poste- 

 riorly, where the thickness is about one-fifth the greatest diameter. 

 Lateral faces gently convex and converging to the acute anterior edge, 

 which is occupied by a single well-defined ridge bearing in places lat- 

 erally compressed closely arranged stellate tubercles, but which in other 

 parts have the appearance of lateral decussations, possibly due to the 

 wearing down of the crest of the tuberculose ridge ; lateral faises of a 

 medium-sized specimen occupied by about fifteen prominent, rounded or 

 flattened, rarely bifurcated costse, which very gradually diminish in size 

 posteriorly until nearing the beveled border, in which the posterior 

 ridges become obsolete descending and equal the anterior ones in size; 

 the anterior costse also bear oblique, closely approximated tubercles, 

 sculptured by radiating ridges, occurring on the third or fourth costte from 

 the margin, the posterior ridges being destitute of ornamentation, except 

 occasional traces of minute tubercles and the delicate longitudinal stria- 



