NO. 2177. FOSSIL FISHES IN NATIONAL ilVSEVM— EASTMAN. 265 



species for the more slender variety, especially in view of the fact that 

 the lower portion of the convex margm is not entire, and some of the 

 posterior denticles have become lost. 



Important to note is the fact that the original of plate 5, figure 2, 

 displays very clearly the line of insertion at the base, and the direc- 

 tion of this line would seem to indicate that in this as in other species 

 of PJiysonemus, the distal portion of tlie spine was forwardly curved. 

 This disposes of the view formerly entertained that the P. arcuatus 

 type of head-spine was homologous with the laterally compressed 

 posterior branch of Erismacantlius (see pi. 7, figs. 2, 3). That this 

 recurved portion of Erismacanthus spines was actually posterior in 

 position follows as a necessary consequence of the mterpretation of 

 these organs as frontal claspers. It is probable that in PJiysonemus, 

 as in Heteracanthus, the forwardly arched spines were situated imme- 

 diately behind the head, at the junction with the neck. 



Formation and locality. — St. Louis limestone; near St. Louis, Mis- 

 souri (from the G. Hambach collection) . 



Genus ERISMACANTHUS M'Coy. 



Of this genus two European and two American species have been 

 described, all of the forms agreeing closely vnih the Physonemus 

 type of frontal spine, but differing from it in that the spines are 

 divaricated ; that is, they consist of two branches extending in oppo- 

 site directions in the same vertical plane. Tlie imperfect ichthyo- 

 doruhtes- known as Gampsacanthus, Lecracantlius , and DijmacantTius 

 appear to be of the same general nature, and may be provisionally 

 regarded as the dissociated anterior branches belonging to Erismacan- 

 tlius. The spines of tliis genus are somewhat asymmetrical, and were 

 probably located on either side of the head region, whereas the 

 bilaterally symmetrical PJiysonemus type of frontal spine occupied 

 an occipital position. 



ERISMACANTHUS FORMOSUS Eastman. 



Erismacanthus formosus Eastman, Amer. Naturalist, vol. 36, 1902, p. 850, text- 

 fig. 1; Bull. Mus. Comp. Zool., vol. 31, 1902, p. 212, text-fig. 13. 



The type of this, the largest known species of the genus, was col- 

 lected by Dr. G. Hambach in the St. Louis limestone of Missouri, and 

 is now preserved in the collection of the United States National 

 Museum. 



ERISMACANTHUS MACCOYANUS St. John and Worthen. 



Plate 7, figs. 2, 3. 

 Erismacanthus maccoyanus St. John and Worthen, Pal. Illinois, vol. 6, 1875, 

 p. 461, pi. 22, figs. 1, 2, 4 {not fig. 3).— Eastman, Bull. Mus. Comp. Zool., 

 vol. 31, 1902, p. 211. 



The paired frontal claspers described under this name are all of 

 small size, none so far as known exceeding 5 cm. in length. One of 



