246 PALEONTOLOGY OF ILLINOIS. 



GENUS EU^EMACA^THUS, 1 St. J. and W. 



Fin-spines arched backward, laterally compressed. Dorsal border 

 rounded and occupied by a single plain, smooth ridge enameled 

 throughout, and perhaps inbeveled along the lateral edges, which 

 may be delicately sculptured. Lateral surfaces bearing longitudinal 

 costae interrupted by transverse ridges. Inter-costal furrows plain, 

 or occupied by irregularly dispersed tubercles. Posterior face trun- 

 cated, the lateral angles bearing a row of denticles whose apices 

 are directed upward. Pulp cavity apparently inclosed throughout 

 the greater length of the exposed portion of the spine. 



The typical and sole representative of the genus here recognized, 

 is the dorsal spine, originally described by Messrs. Newberry and 

 Worthen, under the name Ctenacanthus? costatus, from the Saint 

 Louis limestone. Allied to Ctenacanthus, the genus is especially 

 characterized by the presence of the plain dorsal ridge, the tuber- 

 culated inter-costal sulci, and the upward direction of the denticles 

 arming the angles of the posterior face. The interrupted character 

 of the costae of the lateral surfaces is precisely of the same nature 

 so prevalent amongst though not restricted to the species of Cten- 

 acanthus ; the tuberculose inter-costal grooves and the direction of 

 the apices of the posterior denticles recall a relationship with 

 Asteroptychius, Agass., from which, however, it is distinguished by 

 the transversely ridged and perhaps tuberculose character of 

 the costae. 



EUNEMACANTHUS COSTATUS, (N. and W. sp.) 



PI. XXIII. Fig. 2. 



Ctenacanthus? costatus, NEWBEBBY and WOBTBEN, 1866. 111. Geol. Surv., II, p. 120 

 PI. XII, f.5. 



Spine of medium size, moderately arched along the dorsal edge 

 and somewhat rapidly tapering, laterally compressed, presenting a 

 wedge-shaped transverse section. The anterior or dorsal ridge 

 arched transversely in about a quarter of a circle, lateral edges 

 inbeveled and marked with delicate, sharp raised lines directed 

 obliquely downward, the superior surface enveloped in a polished 

 smooth enamel layer, occupying a space about equal to two of the 

 adjacent lateral costae. The latter irregularly diminished in size 



1. Eu, beautiful; nerna, line; akantha, spine, 



