176 bulletin: museum of comparative zoology. 



base of the crown has several narrow imbricating folds of dentine descending 

 lower on the posterior than on the anterior face. The root is relatively large 

 in typical species, with a tumid and truncated lower extremity, and is longest 

 and broadest in teeth belonging to the symphysial series. The shortness of the 

 root in some species has led to the establishment of such genera as " Antliodus " 

 and " Chomatodus," which are best included under the same head as the more 

 typical forms. 



Although the teeth of Petalodus are scarcely distinct from those of Cten- 

 optychius, as already observed by Tracpiair,* practical reasons render it desirable 

 to retain the former as a provisional genus, and besides, the uniformly entire 

 condition of the cutting-margin in Petalodus seems to be a character of more 

 than specific value. A serrated cutting-margin is simulated only amongst 

 unequally worn teeth, usually belonging to the lateral series, in some species. 

 In Ctenoptychivis the teeth of both upper and lower jaws are distinctly ser- 

 rated, in Peripristis only those of the upper jaw. The teeth of Petalodus are 

 known to have been arranged in series closely similar to those of Janassa, the 

 larger and symmetrically formed teeth occupying a symphysial position, and 

 the lateral series diuiinishing in size, besides becoming more oblicpie, on pass- 

 ing from the center. Jaekel's conjecture that the .symphysial series were of 

 the form known as Petalorhynchus is clearly untenable. 



Petalodus alleghaniensis Lkidy. 



(Plate 2, Figs. 17, 18; Plate .i. Fir. 27.) 



1853. Petalodus ohioensis J. M. Safford, Amer. Journ. Sci. (2), Vol. XVI., p. 142. 



[Insufficiently defined.] 

 1856. Sicarius extinctus J. Leidy, Proc. Acad. Nat. Sci. Pliilad., Vol. VII., p. 414. 



[Insufficiently defined.] 

 1856. Petalodus alleghaniensis J. Leidy, Journ. Acad. Nat. Sci. Philad. (2), Vol. III., 



p. 161, PI. XVL, Figs. 4-10. 

 1866. Petalodus destructor J. S. Newberry and Worthen, Pal. Illinois, Vol. II., p. 35, 



PI. II., Figs. 1-3. 

 1870. Petalodus destructor O. H. St. John, Proc. Amer. Phil. Soc., Vol. XL, p. 433. 



1872. Petalodus destructor 0. H. St. John, Ilayden's Final Kept. U. S. Geol. Surv., 



Nebraska, p. 241, PI. III., Fig. 5. 



1873. Petalodus aller/haniensis J. Leidy, Kept. U. S. Geol. Surv. Territ., Vol. I., p. 



312, PI. XVIL, Fig. 3. 

 1875. Petalodus alleghaniensis J. S. Newberry, Rept. Geol. Surv. Ohio, Vol. II., 



p. 52, Pi. LVIIL, Fig. 13. 

 1875. Petalodus alleghaniensis St. John and Worthen, Pal. Illinois, Vol. VI., p. 396. 



1895. Petalodus securiger 0. P. Hay, Journ. Geol., Vol. III., p. 561, Figs. 1, 2. 



1896. Petalodus alleghaniensis C. R. Eastman, Journ. Geol., Vol. IV., p. 174. 



1899. Petalodus sp. 0. Jaekel, Zeitschr. deutsch. geol. Ges., Vol. LL, p. 287, 

 Fig. 6 J. 



1 Geol. Mag. (3) Vol. V., 1888, p. 85. 



