YEBTEBBATES. 289 



CTENACANTHUS CANNALIRATUS, St. J. and W. 



PI. XXI, Fig. 3. 



Fin-spine long and slender, very gradually tapering, moderately 

 arched along the dorsal edge, much compressed laterally, posteriorly 

 truncated, wedge-shaped in transverse section, pulp cavity small, 

 elongate oval in outline and confined within the posterior half of 

 the diameter of the spine. Posterior face slightly depressed and 

 traversed by a faint median keel, postero-lateral angles sharp, 

 the slightly raised inner border showing indications of moderately 

 spaced, minute, laterally compressed denticles. Lateral surfaces 

 gently convex transversely and regularly converging toward the 

 sharply rounded anterior edge, and occupied by more or less regu- 

 larly rounded costse, intercostal furrows deep, about half the breadth 

 of the ridges. The posterior costse are enveloped in a smooth 

 enamel coating ; toward the front they show delicate flexuous sharp- 

 crested longitudinal lines, and the second or third rib from the 

 dorsal ridge becomes distinctly interrupted by transverse carinse, at 

 the same time retaining the thread-like lines ; the remaining ridges 

 are more and more strongly marked by the transverse carinse, the 

 anterior one strongest of all. The carinae cross the ribs obliquely, 

 culminating above in delicate sharp crests, most prominent along 

 the anterior margin where they are ornamented by a few sharp 

 radiating lines producing a pectinated or notched appearance in 

 that side ; the anterior ridge is prominent, somewhat compressed 

 laterally, and marked by similar slightly asymmetrical transverse 

 ridges, which, however, are equally developed in either margin, pro- 

 ducing a faint undulation along the rounded crest. 



The collection of Dr. Hambach affords a small section of one of 

 the long, slender spines of Ctenacanthus different from any form 

 heretofore derived from the Chester formation. The spine is inti- 

 mately related to Ct. gracillimus, N. and W., of the St. Louis lime- 

 stone, for which indeed it might be mistaken on casual observation. 



However, searching comparisons with the latter species discloses 

 certain details in ornamentation which may indicate for the Chester 

 form a specifically distinct, though closely allied, character. While 

 the form and general appearance of the latter is strikingly like the 

 St. Louis spine, there is a marked dissimilarity in the character of 



