VERTEBRATES. 113 



allel ridges, which arch over from the medial toward the round- 

 ed angle; enamel surface finely and evenly punctate; length 

 1 inch, 3 lines; greatest breadth 7 lines. 



This species may be distinguished from the preceding by its smaller size, by 

 its most elevated point being nearer to one of the lateral angles, by the absence 

 of the ridge running down to the longer side, and by the plication of one of 

 its faces. 



Figure 7 and la, represent the crown as seen from above, and the profile 

 from the side of the medial angle. 



Formation and locality : Keokuk limestone, Nauvoo, Illinois. 



Genus HOMACANTHUS, Ag. 



Gen. Char. — Fin-spine small, rather rapidly tapering, mod- 

 erately arched backwards; sides flattened, converging to the 

 anterior face, which is obtusely keeled; sides covered with few 

 very coarse, longitudinal ridges, and fine striae in the same 

 direction; posterior margin with two rows of denticles arched 

 downwards. 



This genus differs from Leptacanthus and Ctenacanthus in its small size, and 

 the sides being covered with very few, strong, nearly smooth ridges. The genus 

 is by no means a well defined one, but when confined to the two following spe- 

 cies, and the Russian Devonian one, already published, it has a sufficiently dis- 

 tinct facies. 



Homacanthus gibbosus, N. and W. 



PL XII, Fig. 1. 



Spine small, slender, strongly arched backward, narrow and 

 compressed throughout the upper two-thirds of its length, then 

 rapidly expanding toward the base till at the point, when the 

 ornamentation ceases, it is nearly twice as broad as in the mid- 

 dle; immersed base without ornament, somewhat roughened, 

 and irregularly conical about one-fifth of the entire length; 

 exposed portion marked throughout with longitudinal, tuber- 

 —15 



