420 PALAEONTOLOGY OF ILLINOIS. 



Genus CTEIACANTHU8, Agassiz. 



CTEN ACANTHUS SPECTABILIS, St. J. and W. 

 PI. XV, Kg. la, b, c, d, e. 



Fin-spine robust, attaining a length of seven or eight inches, rapidly 

 tapering, the line of insertion indicating a posterior inclination at an 

 angle of about 45°, transverse section sublenticular, truncated behind. 

 Lateral faces of the exposed portion broadly expanded at base, flattened 

 or very slightly arched in the posterior half, more rapidly rounded in 

 front to the obtusely angular and gently arched anterior margin, which 

 bears a prominent, eccentric marginal ridge, from which frequent bifur- 

 cations are sent off on either side, and these again bifurcate descending, 

 each off-shoot being more attenuated and curved posteriorly on approach- 

 ing the posterior margin, forming throughout closely approximated, 

 rounded ridges, of which there are about fifty, counting along the infe- 

 rior margin, and less than half that number two-thirds the distance to 

 the apex; the longitudinal costse are occupied by numerous obtuse, 

 apparently smooth, brightly enameled tubercles, more or less regularly 

 spaced by once or twice their own diameter ; along the anterior margin 

 they present the appearance of closely approximated decussations, 

 apparently the result of abrasion, the entire crest of the ridges being 

 reduced to a smooth polished surface, but towards the base where they 

 are less worn, they present the usual condition of more or less transverse 

 tubercles abruptly rising from the rounded costae, similar to those occur- 

 ring in the posterior rows ; the intercostal furrows are very narrow, in 

 diameter less than that of the ridges, and minutely punctate'or striato- 

 puuctate. Posterior face slightly channeled either side of the low, 

 obtusely angular median ridge ; posterolateral margins forming right 

 angles, and occupied by apparently small, closely arranged denticles. 

 Pulp cavity ovate or sublenticular in section, situated in the posterior 

 two-thirds of the spine, less than one-third the diameter of the spine a 

 third the distance from the apex, but ruuuing out near the line of inser- 

 tion, deeply channeling the postero-inferior margin. Basal portion rap- 

 idly tapering to the proximal extremity, nearly straight in front and 

 gradually rounded behind to the point, surface finely striato-punctate. 



The unique example of the elegant form above described was discov- 

 ered by Dr. E. N. Whipple, of Legrande, Iowa, to whom we are 

 indebted for its possession. It represents a nearly entire fin-spine, per- 

 haps an inch and a half of the distal extremity and one inch of the 

 opposite end are broken off, but otherwise the specimen is in a remark- 

 ably perfect state of preservation. Its great breadth along the oblique 



