NO. 2177. FOSSIL PISHES IN NATIONAL MUSEUM— EASTMAN. 263 



Keokuk species. Stethacanthus seems to have attained its maximum 

 size in the stage represented by the Keokuk hmestone, as Physonemus 

 did in the Burhngton; and a marked diminution of size is observable 

 in both genera toward the close of the Mississippian series. 



A certain group of large Physonemus-like spines is peculiar in that 

 the exserted portion is forwardly curved, mstead of backwardly, as 

 in most ichthyodoruhtes, and this feature appeared so anomalous to 

 early authors, like Leidy, Newberry, and Worthen, as in their judg- 

 ment to warrant a generic separation from Physonemus. Hence sev- 

 eral species belonging to this category were described under the 

 names of Xystr acanthus and Dreixinacanthus. The group of large, 

 forwardly curved spines referred to includes such forms as the so- 

 called Drepanacanthus gemmatus Newberry and Worthen, I), anceps 

 Newberry and Worthen, Xystracanthus acinaciformis St. John and 

 Worthen, Physonemus gigas Newberry and Worthen, and the defenses 

 theoretically associated with the teeth of Polyrhizodus rossicus by A. 

 Inostranzev ^ and O. Jaekel.^ However, it seems preferable to retam 

 all of the "species" represented by these spines within the Hmits of 

 Physonemus, and their forward curvature favors the interpretation 

 of these bodies as head spines and claspmg organs such as are devel- 

 oped among recent and fossil Chimaeroids. 



Following is a list of the known North American species of Physo- 

 nemus, understood in its broader sense : 



1. P. hamus-piscatorius Eastman Isinderhook. 



2. P. pandatus Eastman Kinderhook. 



3. P. gigas Newberry and Worthen Burlington. 



4. P. gemmatus (Newberry and Worthen) Keokuk. 



5. P. stellatus (Newberry and Worthen) Keokuk. 



6. P. (?) baculiformis (St. John and Worthen) Keokuk. 



7. P. (?) necis (St. John and Worthen) Keokuk. 



8. P. arcuatus M 'Coy St. Louis. 



9. P. acinaciformis (St. John and Worthen) Coal Measures. 



10. P. anceps (Newberry and Worthen) Coal Measures. 



11. P. asper Eastman Coal Measures. 



12. P. mirabilis (St. John and Worthen) Coal Measures. 



PHYSONEMUS GEMMATUS (Newberry and Worthen). 



Plates 3 and 4; plate 5, fig. 3. 



Drepanacanthus gemmatus Newberry and Worthen, Pal. Illinois, vol. 2, 1866, 

 p. 123, pi. 12, figs. 1, 2. 



The only pubhshed figures of this species are imperfectly pre- 

 served spines in which the apical portion is lacking, and the inserted 

 basal part is not completely shown. Two very large (25 cm. high) 

 and well preserved spmes belonging to the United States National 

 Museum collection fortunately supplement each other as regards cer- 



1 Travaux Soc. Nat. St. P^tersb., vol. 19, 1888, pp. 1-18, figs. 7-10. 

 « Zeitschr. Deutsch. Geol. Ges., vol. 51, 1899, p. 281, fig. 5. 



