120 PALAEONTOLOGY OP ILLINOIS. 



Ctenacanthus? costatus, N. and W. 



PI. XII, Fig. 5. 



Spine short, robust, slightly arched backward, with an ovate 

 section near the base, elliptical above; basal portion once sunk 

 in the integuments about one fourth of the total length, irreg- 

 ularly pointed, smooth or finely striated; exposed portion 

 coated with polished enamel, strongly ribbed longitudinally; 

 anterior border occupied by a single broad and smooth carina, 

 three times the width of the costse of the sides; lateral sur- 

 faces occupied at base by broad parallel longitudinal ribs sepa- 

 rated by much narrower sulci. The external surface of these 

 ribs is smooth and polished, but their sides are set with closely 

 approximated tubercles which give them an elegant beaded 

 appearance and nearly fill the sulci. 



In the only specimen included in the collection a portion of the posterior 

 surface of the spine is wanting. The number and form of the denticles which 

 once existed there can not, therefore, be fully determined. Above this part 

 are a few tubercles which seem to have formed a double row along the posterior 

 surface of the lower portion of the spine. The ribs are of nearly equal breadth 

 and are continuous throughout, except the two anterior ones, which become 

 obsolete before reaching the summit. 



Until better specimens can be obtained, or such as show the posterior denti- 

 cles more distinctly, a full and satisfactory description can scarcely be made 

 out. It is evident, however, that the specimen before us represents a strongly 

 marked and hitherto undiscribed species, which will be at once recognized by 

 its few, strong, longitudinal costse, plain and smooth in their outer faces, ele- 

 gantly beaded along their sides. 



Formation and locality: St. Louis limestone, St. Louis, Missouri. 



Genus DREPANACANTHUS, N. and W. 



Gen. Char. — Fin-spines of medium or large size, more or less 

 compressed laterally, gradually tapering to an acute point, 

 strongly curved, point turned forward ; anterior margin set with 



