178 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



A single tooth of this species is described by St. John from the Missourian 

 near Nebraska City, and another is figured by St. John and Worthen from a 

 corresponding horizon in Adams County, Iowa. Professor Barbour has also 

 obtained a solitary example from the Atchison shales of Peru, Nebraska. 



Formation and Locality. — Missourian ; Iowa and Nebraska. 



Ctenoptychius OCCidentalis (St. John and Worthen). 



(Plate 2, Fig. lO.) 



1875. Ctenopetalus occidentalis St. John and Worthen, Pal. Illinois, Vol. VI., p. 401, 

 PI. XII., Fig. 14. 



This species is founded on very delicate, gently arched teeth with relatively 

 few (10-12) and obtuse coronal serrations, the broad basal band on the outer 

 face being sharply set off from the exposed portion of the crown. The two 

 examples known to the aiithors of this species were derived from the Lower 

 Coal Measures in the vicinity of Fort Dodge, Iowa. The trivial title of 

 occidentalis was bestowed by the same authors upon still another species of 

 Ctenoptychius, which they placed in the now obsolete genus " Harpacodus." 

 The form occurs in the St. Louis limestone of Illinois and Missouri, and so 

 closely resembles C. compactus (St. J. and W.) from the Chester Group, that 

 we have no hesitation in uniting it with that species. A single detached 

 crown of G. occidentalis, somewhat weathered, was obtained by Professor 

 Barbour from the Atchison shales of Richfield, Nebraska. 



Formation and Locality. — Productive Coal Measures ; Iowa. Missourian ; 

 Nebraska. 



PERIPRISTIDAE. 



PBRIPRISTIS St. John. 



Proc. Amer. PhiL Soc, Vol. XL, 1870, p. 434. 



Hoplodus n. Etheridge, Jan., Geol. Mag. (2), Vol. II., 1875, p. 243. 

 Diodontopsodiis J. W. Davis, Brit. Assoc. Kept., 1881, p. 640. 

 Pristodits J. W. Davis (ex Afassiz MS.), Trans. Roy. Dublin Soc. (2), Vol. L, 

 1883, p. 519. 



Peripristis semicircularis (Newberry and Worthen). 

 (Plate 3, Figs. 5-7 ; Plate 3, Fig. 25.) 



1866. Ctenoptychius semicircidaris Newberry and Worthen, Pal. Illinois, Vol. II., 



p. 72, PI. IV., Fig. 18. 

 1870. Peripristis semicircularis O. H. St. John, Proc. Amer. Phil. Soc., Vol. XL, 



p. 434. 



