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 Traquair,! 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 seemseto 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 Ctenoptychius 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 diminishing in size, besides becoming more oblique, 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 Leripy. 
(Plate 2, Figs. 17, 18; Plate 3, Fig. 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. Philad., Vol. VIL, p. 414. 
[Insufficiently defined. ] 
1856. Petalodus alleghaniensis J. Leidy, Journ. Acad. Nat. Sci. Philad. (2), Vol. IIL, 
p. 161, Pl. XVI, Figs. 4-10. 
1866. Petalodus destructor J. S. Newberry and Worthen, Pal. Illinois, Vol. IL., p. 35, 
Pl. IL., Figs. 1-3. 
1870. Petalodus destructor O. H. St. John, Proc. Amer. Phil. Soc., Vol. XI., p. 483. 
1872. Petalodus destructor O. H. St. John, Hayden’s Final Rept. U. S. Geol. Surv., 
Nebraska, p. 241, Pl. IIL. Fig. 5. 
1873. Petalodus alleghaniensis J. Leidy, Rept. U.S. Geol. Surv. Territ., Vol. I. p. 
312, Pl. X VIL. Fig. 3. 
1875. Petalodus alleghaniensis J. S. Newberry, Rept. Geol. Surv. Ohio, Vol. IL, 
p..62, Fl LVEML, Big: te. 
1875. Petalodus alleghaniensis St. John and Worthen, Pal. Illinois, Vol. VI., p. 396. 
1895. Petalodus securiger O. 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. O. Jaekel, Zeitschr. deutsch. geol. Ges., Vol. LI, p. 287, 
Fig. 6A. 
1 Geol. Mag. (8) Vol. V., 1888, p. 865. 
