VERTEBRATES. 119 



near the anterior border pectinated by very numerous trans- 

 verse edges, which on the posterior carinse become small, dis- 

 tinct, rounded tubercles ; posterior half of the lateral surfaces 

 marked with fine, and interrupted longitudinal striae; posterior 

 surface finely striated longitudinally; striae reticulated; poste- 

 rior angles set with numerous, small, depressed hooks, separa- 

 ted by about the width of their bases. 



This is a peculiarly exact and elegant little spine, of which the surface is 

 every where highly ornamented. Its most striking character is perhap.3 its 

 angularity, but the ornamentation is peculiar, and such as will readily serve to 

 distinguish it from any species hitherto described. The anterior surface is 

 occupied by relatively large and distinct costsc, strongly pectinated, as is com- 

 mon in the genus, while about half of the lateral surfaces, and all the posterior 

 sulci, are marked by fine, longitudinal, interrupted, or reticulated striae. Near 

 the base the greater part of the lateral surface is covered by the pectinated or 

 tuberculated ridges, while toward the summit the striae encroach upon the 

 costae. and occupy most of the side. The only specimen in the collection, 

 though apparently representing nearly the entire length of the spine, wants 

 the extreme summit, and that part of the base which was buried in the integu- 

 ments of the back. It is 3J inches long, by 6 lines wide. The lateral parietes 

 of the central cavity near the base were evidently quite thin, and have been 

 crushed in by pressure from without. 



It will be noticed that both this and the preceding species referred to the 

 genus Ctenacanthus are all small and delicate in form and markings, contrast- 

 ing strongly with many species of the genus which have been described, espe- 

 cially with the large, nearly cylindrical spines, described by Agassiz and Eger- 

 ton under the names of G. major, G hybodoides and G. nodosus, and we may 

 justly doubt whether the long, slender, and delicate spines such as G. distans, 

 McCoy, and G. gracillimus, Nob., were borne by fishes generically identical 

 with those which carried the formidable weapons named above. While, how- 

 ever, we have only the spines with which to classify these ancient placoids, and 

 the differences which they present are rather of degree than kind, we must 

 be content to group together those which are generally similar, waiting the 

 time when the spines and teeth can be found together, and their generic rela- 

 tions more accurately established. 



Figures 4, 4 a, side and posterior faces, natural size. 



Formation and locality: Chester limestone, Chester, Illinois. 



