424 PALAEONTOLOGY OF ILLINOIS. 



Ctenacanthtjs speciosus, St. J. and W. 



PI. XIV, Tig. 3, 4. 



The present form, as indicated by a large and nearly perfect fin-ray 

 discovered by Mr. Waohsmuth, bears, in its rapidly tapering outline 

 as seen from the side, much resemblance to G. spectabilis, perhaps even 

 a stronger likeness to C. varians, in the above and other respects. The 

 anterior margin is gently arched and rounded, from which the lateral 

 faces very gradually diverge in a slight curvature to the posterior mar- 

 gin in the upper portion, but apparently very slightly converging in the 

 lower part, as shown in the transverse section, which above is wedge- 

 shaped and below long-elliptical, truncated behind. Posterior face 

 nearly plane near the apex, the median ridge gradually increasing in 

 strength and quite prominent below, gently channeled either side and 

 limited by the well-defined postero-lateral angles, which bear a row of 

 low, broad, obtusely pointed denticles, spaced by once or twice their own 

 diameter, and which are probably confined to the upper third of the spine. 

 Line of insertion presenting a slight sigmoidal curvature, very oblique, 

 the posterior limb being almost parallel with, or diverging at an angle 

 of 5° to 10° from the anterior margin. Successive lines of growth, so 

 to speak, of less and less obliquity the further removed from the pres- 

 ent dorsal line, seem to show that in the development of the spine acces- 

 sions to its substance were more abundantly supplied to the deuse 

 anterior portion ; hence the line of insertion, as exhibited in individuals 

 of different stages of growth, would indicate various degrees of incli- 

 nation, a young spine occupying a relatively more erect position than is 

 the case in the mature condition — a fact which is probably equally 

 applicable to all ichthyodorulites of this class. The basal portion 

 extends half way to the apex in the posterior margin, broadly rounded 

 to the inferior extremity, exterior surface delicately punctate or striato- 

 punctate, deeply excavated behind by the pulp cavity, which latter 

 occupies the posterior two-thirds of the diameter of the spine above. 

 The exposed portion of the spine is ornamented by the most exquisite 

 and varied sculpturing; the anterior edge is occupied by a more or less 

 eccentric ridge, slightly more bulky than the others, which rarely bifur- 

 cates until nearing the basal line, and this is true of all longitudinal 

 ridges occupying the body of the spine, which send off most frequent 

 branches toward their bases in the expanded portion of the spine, where, 

 in a mature individual, they number above eighty; very irregular in 

 size and arrangement, closely crowded, separated by interspaces rarely 

 half their diameter, the ridges present the appearance of minute decus- 



