VERTEBRATES 401 



The intimate relations which exist between Ctenojpetalus and Petalodus 

 find an additional exemplification in the present form. Indeed, it would 

 appear that the non-serrate forms of the one pass by almost imperceptible 

 gradation into the serrate condition of the other : while in general form 

 the species of both groups are most intimately related. 



Position and locality : Uncommon in the Chester limestone ; Chester, 

 Illinois. 



Ctenopetaltts occidentalis, St. J. and W. 



pi. xir, Kg. n. 



Teeth small, symmetrical. Crown moderately thick below, rapidly 

 attenuated towards the crest, which is slightly acuminate and broadly 

 arched, with ten to twelve obtusely rounded denticulations, which more 

 or less regularly diminish in size towards the lateral extremities, the 

 median one forming the apex being the most conspicuous ; concave 

 crown-face long-oval in outline, more or less sharply rounded at the 

 extremities, rather deeply and uniformly depressed, and limited below 

 by a relatively narrow, slightly produced basal band, which is appa- 

 rently composed of not more than three or four imbrications, gently 

 aud regularly arched downward in the middle and slightly deflected 

 laterally ; the opposite face is more elliptical or sublenticular in outline, 

 moderately convex iu both directions, and acutely rounded at the 

 extremities, in hight above the basal angle, about half that of the con- 

 cave face ; the basal band in the convex face is remarkable for its great 

 width, the uppermost fold being usually narrow, but the lower fold, or 

 folds, reaching far down and investing nearly half the vertical extent 

 of the inferior basal area in a thin coating of enamel ; in the downward 

 curvature somewhat more sharply arched thau the opposite border with 

 which it nearly corresponds. The base is of moilerate dimensions in 

 proportion to the crown, slightly eccentric, lateral angles and basal 

 margin uniformly rounded, thickened, lateral margins diverging to the 

 crown, gently beveled along the inferior convex border, more or less 

 roughened. The coronal surfaces present a firm smooth enamel-like 

 layer, which is usually more or less abraded along the crest and deli- 

 cately marked with slightly diverging striae. Greatest lateral diameter 

 of crown .66 inch, hight of tooth .52, elevation of concave crown-face 

 .35, hight of convex crown-face above basal angle .24 inch. 



The above species is known from two examples, one of which exhibits 

 a nearly entire specimen, which were discovered by Mr. W. R Morley 

 in strata near the base of the Coal Measures The species falls uuder 

 the genns Ctenopetalits of Aga u siz, and is intimately allied to the typical 



