404 PALAEONTOLOGY OF ILLINOIS. 



five imbricating folds, of which the uppermost ones are curved upward 

 and lost in the smooth surface, basal border gently produced and gently 

 beveled, sometimes abruptly, to the basal region ; the convex face is 

 usually slightly less elevated than the concave face, moderately convex 

 in both directions, basal augle very obtuse, nearly horizontal, or viewed 

 obliquely from below appearing gently arched upward in the middle, 

 more or less sharply rounded at the lateral extremities, gently beveled 

 to the base, the beveled area rather wide and occupied by at least three 

 imbricating folds, continuous with the opposite coronal belt ; coronal 

 region invested with a dense, polish enamel-like layer. Base propor- 

 tionately strong, obliquely produced, usually as broad and deep as the 

 crown, eccentric, compressed beneath the basal margins of the crown, 

 expanded and thickened below, lateral angles sharply rounded, beveled 

 in the deeper face to the broadly rounded, obtuse inferior margin, sur- 

 faces more or less roughened. A small and unusually symmetrical 

 specimen measures in hight .18 inch, greatest lateral diameter at base 

 of crown .13, hight of concave crown face .12, hight of convex face 

 nearly .10 inch. 



The only two representatives of this interesting species at present 

 known to us, were obtained at the same locality — indeed so intimately 

 associated that the one was discovered in developing the other from the 

 shaly matrix — in a stratum which has afforded no other Petalodont forms 

 or other ichthyic remains, except Petrodus and scales of Palceoniscus. 

 The smaller and more symmetrical specimen is that upon which the pre- 

 ceding description was based, but the other tooth exhibits such marked 

 individual variation from what may be regarded as the normal aspect 

 of the species, that it is not merely a matter of interest but important 

 carefully to note the variations so far as may be necessary for the satis- 

 factory definition of the species. The individual teeth are subject to 

 considerable variableness in the symmetry of outline and relative pro- 

 portion of parts. The large specimen presents a more distorted outline 

 than the small tooth, the base being strongly deflected, without, how- 

 ever, losing its general shape; the crown is conical, both surfaces very 

 full or arched laterally, and nearly equal in hight, apex conical, laterally 

 deflected, and slightly curved over the concave face, lateral edges of the 

 the crest very obtuse, scarcely defined, basal margin slightly arched 

 downward in the middle and beveled gently to the basal region, imbri- 

 cated belt very wide and composed of three or four continuous, though 

 irregularly disposed folds, with several secondary thread-like folds 

 reaching two-thirds the distance to the apex. This tooth is unquestion- 

 ably specifically identical with that first noticed, differing chiefly in the 

 less distinct definition of the concave crown face and lateral obliquity of 

 the base. The species differs so widely from other Petalodont forms as 



