VERTEBRATES. 429 



relatively prominent with indications of strong, closely arranged denti- 

 cles near the upper extremity. Pulp cavity, above, occupying about 

 one-third the diameter of the spine, broadly ovate in section. Base 

 unknown. 



We have had opportunity to examine only two imperfect specimens 

 of the present species, but these exhibit such strongly marked peculiar- 

 ities as to preclude any hesitation in recognizing their specific distinct- 

 ness. The specimens show respectively one and two inches of the upper 

 extremity, in the one instance of a fairly preserved individual, the other 

 or larger fragment being considerably abraded along the anterior ridge, 

 which presents the appearance of a broadly rounded, perfectly smooth 

 surface — the costas in the lateral faces also bear evidence of wear, which 

 has nearly obliterated their decussated or tuberculose ornamentation. 

 The smaller spine exhibits the superficial characters in a very satisfac- 

 tory manner, though here some allowance must be made on account of 

 wear, but to a much less extent than in the former case. Thecostseare 

 very prominent, slightly inbeveled laterally and broadly rounded along 

 the polished crest, which is crossed by - delicate thread-like ridges or 

 decussations which terminate laterally in more promiuent projections, 

 and which cross the costae in varying degrees of obliquity or arched 

 upward, three or four occurring in the space of one-tenth inch. 



In outline and general proportions, it resembles G.Mayi of the Upper 

 Burlington limestone ; it is, however, proportionately thicker, the costae 

 narrower and more widely spaced. 



Position and locality : In the upper fish-beds of the Keokuk lime- 

 stone : Bentonsport, Iowa ; La Grange, Missouri. 



Ctenacanthtjs gemma tits, St. J. and W. 



PI. XV, Kg. 9,10. 



Fin-ray long, gradually tapering, gently curved, lenticular in trans- 

 verse section, abruptly truncated posteriorly, the posterior face nearly 

 plane either side of the narrow median keel, meeting at the postero- 

 lateral borders in a right angle. The lateral faces gradually converge 

 in a slight convexity to the anterior margin, which presents a narrow, 

 prominent, sharply rounded ridge, regularly crossed by beautifully 

 sculptured tubercles, which meet in an obtuse angle along the crest, the 

 swollen lateral extremities being gently directed downwards. Lateral 

 cost&e sixteen, more or less, in either face, prominent, more or less 

 regularly spaced by about their own diameter, very gradually, though 

 somewhat irregularly, diminishing in size posteriorly, and ornamented 

 with elegantly wrought stellate tubercles, which form in the anterior 

 costs; transverse or slightly oblique prominences, but less prominent 



