VERTEBRATES. 399 



Agass. But as tins serrated character is not apparently constant — even 

 jjortions of the crown in the same individual exhibiting both the plain 

 and serrated conditions of the crest, and in the larger tooth the entire 

 crest is even — it may not be regarded as au important character. Nor 

 does it appear that in the case of Ctenopeialus serratusoi the European 

 Carboniferous deposits the serrated crest should be considered as its 

 most important generic character, since a series of that species, which 

 we have examined in the collection of the Museum of Comparative 

 Zoology, is sure to lead by almost imperceptible gradations from indi- 

 viduals in which the crest is strongly serrated to forms in which the 

 coronal serrations are scarcely more prominent than is the striatiou in 

 Petalodus Icevissimus, Agassiz, with which they are associated. The 

 collection of Mr. Van Horne, from Alton, affords a single imperfect 

 specimen apparently referable to the present form, in which the den- 

 ticulated character of the crown is well displayed. 



Position and locality : In the upper beds of the St. Louis limestone ; 

 Pella, Iowa, and Alton, Illinois. 



Ctenopetaltjs limatultjs, St. J. and W. 



pi. xn, Fig. is. 



Teeth very small, strong and symmetrically proportioned. Crown 

 moderately thick below and gradually attenuated towards the crest, 

 which is moderately arched, acuminate, and somewhat deeply serrated, 

 with as many as seven strong, subtumid, subacute denticulations, of 

 which the median one is strongest, the lateral ones regularly decreasing 

 in size towards the extremities, aud defined from one another by rather 

 deeply impressed sulci extending nearly to the basal line in the convex 

 face ; convex face slightly arched laterally, nearly plane vertically, basal 

 angle well-defined, broadly arched downward in the middle and again 

 gently curved downward at the extremities, and marked upon the 

 inferior or beveled surface by a relatively broad coronal belt apparently 

 without imbrications ; concave face apparently more regularly oval in 

 outline, but slightly depressed, basal margin broadly arched downward, 

 coronal folds unknown, both surfaces smooth aud polished. Base strong, 

 constricted above, slightly expanded and thickened below and broadly 

 ronnded, in depth equal to the hight of the crown, but considerably 

 less in lateral diameter, more or less roughened. Lateral diameter of 

 crown .21 inch, elevation of convex face .10, entire hight of tooth .23, 

 lateral diameter of base .17 inch. 



This elegant little form is intimately allied to G. dentatus (Agassiz 

 sp.) of the British Mountain limestone, with authentic specimens of 

 which, as also the type belonging to the collection of Lord Enniskillen, 



