36 PALAEONTOLOGY OF ILLINOIS. 



The general form of the crown varies much in some instances; as in that 

 represented in Fig. 3, it is only half the usual height. In other respects, no 

 difference is discernable between that and the others; and there is scarcely 

 room for a doubt that both forms belonged to the same species, possibly to the 

 same individual, the difference being due to the place held by the different 

 teeth in the series, with which each jaw was furnished. 



The striation of the edge is usually very distinct and regular. It is pro- 

 duced by the exposure of the calciguous tubes of the enamel, which are 

 arranged in a series directed toward the edge, and are here much larger than 

 elsewhere. In unworn teeth the striae are very short, the tubes of the anterior 

 and posterior surfaces being exactly opposite; their points of junction forming 

 depressions on the edge. The crenulation thus produced, is, in young teeth, 

 beautifully regular; in older and worn ones it becomes irregular and at length 

 obsolete. When the polished enamel coating of the anterior and posterior 

 faces is worn off, a dotted surface is exposed similar to that of Psammodus. 

 This appearance is due to the cutting across of the calciguous tubes, which 

 appear to be set here at right angles to the surfaces. These dots are particu- 

 larly noticeable on the basal folds in worn specimens. 



The interior of the tooth is quite dense and hard, but exhibits an irregular 

 cellular structure. The root is more spongy, and in some cases seems to have 

 been imperfectly ossified. 



The described species of Petalodus, to which this is most like, are P. acumi- 

 natum Ag., (Poiss. Poss. Vol. 3,j>p. 108, 174, T. 19,/. 11-13), {McCoy, Brit. 

 Palaeozoic, Poss. p. 635, PL 3 g, Fig. 4), and P. Alleghaniensis, Leidy. (Jour. 

 Acad. Nat. Sci. Phila., 2d series. Vol 3, p. 161, PI 16,/. 4 ; 5 and 6.) The one 

 from the Carboniferous limestone of England, and the other from the Coal 

 Measures of Pennsylvania. From the first it differs in the greater relative 

 height of the crown, and the longer and narrower root. From the second it is 

 possibly not specifically distinct, as we only know that through the single speci- 

 men figured and described by Dr. Leidy. If that specimen is a fair represen- 

 tation of P. Alleghaniensis, our species may be distinguished from it by its 

 crown, which is more pointed above, and much less arched laterally; and by 

 the root, which is longer and narrower. It should be said, however, that we 

 have every reason to believe that the teeth of Petalodus, from the different 

 portions of the mouth, are considerably different in form, and it is not at all 

 unlikely that the discrepancies between Dr. Liedy's specimens and ours will 

 prove to be insufficient grounds on which to establish a specific distinction. 



Figures 1, 1 a, and 2, 2 a, represent anterior and posterior views of the 

 crowns of two mature teeth ; 2 b, the root anterior surface ; 2c, profile section; 

 Fig. 3, a small and low crown anterior face, all natural size. 



Formation and locality: Coal Measure limestones of Springfield, La Salle 

 and Sugar creek, Illinois. 



