VERTEBRATES. 305 



bordered by a prominent basal band composed of five to seven narrow, 

 regular imbrications, strougly arched downward in the middle and 

 making a slight sigmoidal curvature towards the extremities, sharply 

 iubeveled to the base; the convex face is long-elliptical or sublenticular 

 in outliue, in hight less than that of the opposite face, gently arched 

 laterally, basal line broadly aud regularly curved downward in the 

 middle, and marked in the deeply beveled inferior margin by a rather 

 wide imbricated belt which is made up of irregularly interrupted, deli- 

 cately wrought folds, and which is distinctly defined from the base 

 proper by a faint raised line; both coronal surfaces are invested with a 

 smooth, polished enamel-like layer, which becomes delicately striated 

 along the crest by the exposure of the vertically disposed medullary 

 tubes in the trenchant triturating edge. The base is relatively thick, 

 thickening below aud more strongly beveled beneath the concave face, 

 broadly rounded below, with obtusely defined lateral angles, aud slightly 

 diverging to the base of the crown, in width about two thirds that of 

 the crowu, and nearly equal to the hight of the crown, coarsely rough- 

 ened vertically. A large specimen measures iu lateral diameter of crowu 

 1.65 inch, hight of tooth 1.14 inch, greatest elevation of convex crown- 

 face .55, greatest elevation of concave face about .<>5 inch. 



This form bears a marked resemblance to P. linguifer, aud iu some 

 particulars it also recalls in a striking manner P. curtus. Indeed, so 

 marked is this intermediate relationship both as to structure and occur- 

 rence iu time, it might readily be assumed that they were derived by 

 processes of evolution one from the other. In the acuminate outline of 

 the crown, it resembles the Chester form first mentioned ; but the basal 

 margin of the concave face is suddenly arched downward, producing a 

 median angulation not observed in that form; also the basal belt of the 

 opposite face is composed of much finer and more numerous imbrica- 

 tions, the crown is proportionately less massive; the base is less deep, 

 wider and more angular in the lateral margins — in this latter particular 

 intimately resembling the base of P. curtus of the Warsaw beds. The 

 crown differs from the latter form in its long-elliptical outline, acuminato 

 crest, more nearly equal elevation of the coronal surfaces, as also in the 

 character of the basal belts, and the crown is less deeply concave than 

 is the case in either of the forms with which the present one is compared. 



Pofition and locality : Iu the upper beds of the St. Louis limestone; 

 Alton, Illinois. 



Petalodus pkoximus, St. J. and W. 



PI. XII, Fig. 11 



Under the above name we have designated a peculiar form of teeth, 

 of which we have seen no absolutely entire individuals, but of which 



