VERTEBRATES. 397 



been transferred from Petalodus to Ctenopfychius, and finally became to 

 be regarded as the type of the present group, under the name Ctenope- 

 talus serratus, which was published in the widely distributed list of type 

 specimens in the well-known collection of Lord Enniskillen.* The 

 genus has certain strongly marked facies, as the serrated or denticu- 

 lated crest, which serve to distinguish it from Petalodus, with which it 

 otherwise bears a much more intimate resemblance than to Gtenoptychius , 

 indeed holding the same relationship to Petalodus as the latter genus 

 does to Antliodus. 



The present form attains medium size. Convex face of crown sub- 

 lenticular in outline, gently arched laterally, and quite plane vertically, 

 moderately thick, strong, acutely rounded at the. produced lateral 

 extremities, which are slightly curved downward at one or other, or 

 both extremities, crest very gently arched along its greater length, sud- 

 denly curved downward on nearing the extremities, somewhat obtusely 

 and minutely denticulated, the denticulations becoming strong and 

 sub angular toward the lateral margins, where they are quite strongly 

 developed; lower margin defined by a strong imbricated basal band, 

 consisting of at least three faintly crenulated folds, broadly and regu- 

 larly arched downward in the middle, gently deflected laterally, and 

 relieved at a sharp angle from the coronal surface above, more or less 

 inbeveled below ; the concave face apparently presents an elliptical 

 outline, gently concave in both directions, basal margin not shown; 

 both coronal surfaces are enveloped in a dense, polished enamel-like 

 layer, towards the crest faintly marked with numerous slightly diverg- 

 ing striae, which mark the course of the medullary tubes which termi- 

 nate above singly, or in pairs, or threes, forming the denticulations. 

 Base unknown. Greatest lateral diameter of crown .95 inch, elevation 

 of convex face .36. 



This elegant form is represented by a unique specimen in the collec- 

 tion of Mr. Springer, which was obtained from the ichtbyic horizon 

 in the upper part of the Keokuk limestone. The generic reference is 

 based upon the coronal characters alone, since the base and basal bor- 

 der of the concave crown-face unfortunately are not preserved. But 

 judging from the superficial characters presented by the crown, we have 

 little or no hesitation in regarding it generically identical with the forms 

 recognized under the above generic title, whether or not this group is 

 sufficiently distinct from Petalodus to be regarded as a well-defined 

 genus. Specifically, however, .the present form is readily distinguished 

 by its laterally elongated outline and slightly arched crest. In shape 

 it is nearest the form from the St. Louis limestone, P. (Ctenopetalus) 

 beUulus. which we have provisionally referred to this group ; but its 



* Alphabetical Catalogue of the Type Specimens of Fossil Fishes in the Collection of the Earl of 

 .Enniskillen, at Florence Conrt. Geol. Mag., vol. vi, Dec, 1869. 



