EASTMAN : CARBONIFEROUS FISHES FROM THE CENTRAL WEST. 177 



A large series of this exclusively Upper Carboniferous species have come 

 under the writer's observation, a study of which shows a wide range of varia- 

 tion to exist between the hirge, syiunietrically formed, sympliysial teeth, such 

 us is represented in PI. 2, Fig. 17, and the low-crowned, short-rooted, often 

 ([uite asymmetrical teeth belonging to the outermost of the lateral series. The 

 form of the root is even more variable than that of the crown, as one may 

 satisfy himself by comparison of Figs. 17 and 18 of Plate 2, or the other figures 

 of this species contained in the literature. Dr. O. P. Hay has commented on 

 the fact that in Newberry and Worthen's figures of P. destructor the lateral 

 angles of the crown are acutely terminated, and notes that in the specimen 

 named by him P. securiger they are rounded off, at which point the enamel 

 folds become flexed upward. This appears to be the normal condition mani- 

 fested by all perfectly preserved teeth, but the root being extremely attenuated 

 close to the lateral angles, the edges are rarely found entire. And it is perfectly 

 evident from Newberry and Worthen's Figs. 1-3 that none of the lateral angles 

 in their specimens have escaped injury. 



The imbricated enamel folds at the base of the crown are sometimes distinctly 

 raised on both faces, and usually appear smoother on the anterior than on the 

 posterior face, as if from contact with adjacent older teeth of the same series. 

 The extent to which the teeth of a single series overlapped one another seems 

 to have been greater than in Janassa, and equals that in Petalorhynchus ^ 

 and Ctenoptychius.2 Marks of wear also seem to show that the upper and 

 lower dentition interlocked by a comparatively small margin. 



Besides the single tooth of this species described by St. John from the Mis- 

 sourian of Rock Bluff, Nebraska, numerous examples have been obtained by 

 Professor Barbour from the same formation at Richfield and Table Rock, and 

 from the Permo-Carboniferous of Roca, Nebraska. 



Formation and Locality. — Coal Measures ; Pennsylvania, Ohio, Illinois, 

 Iowa, Nebraska, Arkansas. Pernao-Carboniferous ; Nebraska. 



Petalodus (Chomatodus) arcuatus (St. John). 



1870. Chomatodus arcuatus 0. H. St. John, Proc. Amer. Phil. Soc., Vol. XL, p. 435. 

 1872. Chomatodus arcuatus 0. H. St. John, Hayden's Final Kept. U. S. Geol. Siirv. 



Nebraska, p. 243, PI. VI., Fig. 14. 

 1875. Chomatodus arcuatus St. John and Worthen, Pal. Illinois, Vol. VI., PI. X., 



Fig. 23. 



Low-crowned teeth of the form commonly ascribed to Chomatodus (pars) in 

 all probability represent the pcstero-lateral series of Petalodus, and it would 

 seem that this genus possessed a larger number of tranverse series than Janassa, 

 as well as a more elongated mouth-cleft. The narrow, ray-like mouth-cleft in 

 Janassa is regarded by Jaekel as evidence of specialization. 



1 Cf. J. W. Davis, On the fossil Fishes of the Carboniferous T,imestone Series, 

 Trans. Roy. Dublin Soc. (2), Vol. I., 1883, p. 426, PI. LXI., Fig. 16. 



2 Cf. St, John and Worthen, Pal. Illinois, Vol. VI., 1875, PI. XII., Fig. 9. 



VOL. XXXIX. — NO. 7 2 



