VERTEBRATES. 427 



tion ; intercostal spaces varying from half to nearly the full diameter of the 

 costse and striated similarly. Posterior faces gently depressed and 

 rather coarsely striato-punctate, median carina forming a low, obtusely- 

 angular ridge, and laterally denned by the somewhat prominent, com- 

 pressed postero- lateral angles, which are armed by moderately promi- 

 nent, slightly hooked, laterally compressed denticles, which are regu- 

 larly spaced in the same individual, but varying in the different speci- 

 mens in the ratio of one to three, apparently more crowded below than 

 near the extremity. Iuternal cavity of moderate size, transverse sec- 

 tion as represented in the illustrations. Base unknown. 



The form here referred to occurs quite common, but unfortunately, 

 the specimens are generally in an unsatisfactory state of preservation ; 

 they indicate spines of six to eight inches or more in length, but it is 

 seldom a specimen is found which exhibits the tuberculose ornamenta- 

 tion, though the cost* are well shown in nearly all. Compared with G. 

 Keokuk, its nearest ally and representative species in the next succeed- 

 ing formation, it is distinguishable by the perceptibly less curvature, 

 more numerous or closely approximated denticles, and more compressed 

 postero-lateral angles, the tubercles of the anterior ridge more com- 

 pressed laterally, often thereby giving rise to a sharp serrated crest, 

 and the relatively larger size of the tubercles of the first two or three 

 costse on either side, the direction of their obliquity being apparently 

 the opposite or from left to right. The specimens figured (PL XV, fig. 

 6, 7,) belong to the fine collection of Mr. Springer. 



Position and locality : A common fine-spine in the fish-bed of the 

 Upper Burlington limestone, at the various localities of its exposure in 

 Louisa, Des Moines and Lee counties, Iowa ; Quincy, and Henderson 

 county, Illinois. 



Ctenacanthtts Keokttk, St. J. and W. 



PI. XV, Fig. 8 a, b, o, d, e. 



Fin-spines eight to ten inches, more or less, in length, gently arched 

 and very gradually tapering, lateral faces flattened behind and beveled 

 to the postero-lateral borders, very gradually converging towards the 

 front and somewhat rapidly rounded on approaching the anterior edge, 

 in transverse section long-elliptical, truncated posteriorly. Anterior 

 rib moderately prominent and marked by somewhat tumid, radiate- 

 sculptured tubercles, spaced by about their own diameter ; lateral costse 

 twelve to sixteen in either face, equally prominent, rarely bifurcating 

 descending and still more seldom implanted, very gradually and more 

 or less irregularly diminishing in size posteriorly, in some examples the 

 middle ridges most prominent, the posterior ridges terminating descend- 



