EASTiMAN : CAKB0NIFP:U0US FISHES FROM THK CENTRAL WEST. 217 



been considerably removed \yy chemical decomposition, yet where the surface 

 is preserved it is seen to be covered with vaacuhir impressions extendiiiL; well 

 up toward the superior margin, thus indicating that the spine was deeply 

 implanted. 



Formatvm and Locality. — Kinderhook ; Iowa. Waverly sandstone ; 

 Michigan. 



Stethacanthus erectus, sp. nov. 



(Plate 3, Fig. 29.) 



Type. — Imperfect spine ; United States National Museum. 



A species of about the same size as S. depresses, and differing from it in hav- 

 ing the concave margin more abruptly curved, the exserted portion rising into 

 a narrow and acuminate apex. 



It is unfortunate that the specimen selected as the type of this species is not 

 more perfectly preserved, as it seems to represent an intermediate stage in the 

 modifications affecting the exserted portion of these peculiar spines, being in 

 fact transitional between low forms like S. compressus, S. carinatus, S. depressus, 

 etc., and those in which the apex is recurved, like S. proclivus and .S'. altoncnsis. 

 The anterior shoulder and greater part of the basal portion of this unique 

 specimen have been broken away, but it is probable that the complete outline 

 would show that the base was produced for some distance posteriorly beyond 

 the hinder wall of the summital portion, as in S. altonensis and some other 

 species. The total height of the part pjreserved is 3 cm., thus indicating a 

 species of about the same si/e as the preceding. 



Formation and Locality. — Kinderhook limestone ; Iowa. 



CESTRACIONTIDAE. 



HOMACANTHUS Agassiz. 



This genus, which is evidently closely akin to Ctenacanthus, is thus defined 

 by A. S. Woodward : "Dorsal fin-spines of small size, slender, more or less 

 arched, laterally compressed, and gradually tapering distally; sides of exserted 

 portion ornamented with few, large, smooth, widely spaced longitudinal ridges ; 

 a similar ridge also forming a large anterior keel ; posterior face with a double 

 series of large, downwardly curved denticles." Tlie only American species 

 that have been referred to Homacanthus have since been removed to other 

 genera, but true representatives of this genus are apparently indicated by the 

 spines described in the following paragraphs. 



