VERTEBRATES. 115 * 



ones only being distinctly depressed. These characters would seem to be suffi- 

 ciently marked to justify us in considering them as distinct from Uomacanthus, 

 and as types of a new genus. From the smaller amount of material, however, 

 now in our hands, we have refrained from adding another to the already long 

 list of ill-defined genera of fossil fishes, and have placed them provisionally in 

 Homacantlms. 



Figure 1, side view, natural size. 



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



Homacanthus? rectus, N. and W. 



PL XII, Fig. 6. 



Spine small, slender, straight, or very slightly arched, com- 

 pressed, with an elliptical section; immersed portion about one- 

 sixth of the entire length, narrowed, compressed laterally, hav- 

 ing a triangular section, rapidly drawn to a rude point below; 

 exposed portion marked by 6-7 rounded longitudinal costae on 

 either side, which are set with remote rounded tubercles, 

 arranged in lines descending obliquely from the anterior bor- 

 der, and being separated three times as far longitudinally as 

 laterally in the middle of the spine. The spaces between the 

 costee are of less than equal breadth with them, and are 

 obscurely striated longitudinally. The only specimen in the 

 collection does not fully give the form and arrangement of the 

 denticles of the posterior border, those of the upper portion 

 alone being visible. These are, however, of unequal size and 

 relatively very large, being nearly as long as the diameter of 

 the spine where they are set. 



In the great size of some of the denticles this species resembles II. macrodus, 

 McCoy, ( Op. cit., p. 632, pi. 3 K, Jig. 20,) but that species is much more curved, 

 broader, and has fewer longitudinal costse. 



Figure 6, side view, natural size. 



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



