206 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
branches. The Burlington species of Physonemus and Stethacanthus display 
a marked increase in size, but they are feebly ornamented, and remain so 
throughout the stage represented by the Keokuk limestone. Stethacanthus 
seems to have attained its maximum size in the Keokuk Group, as Physo- 
nemus did in the Burlington, and a considerable falling off in this respect is 
true of both genera in the St. Louis division. The spines of Stethacanthus 
remain unornamented from the time of their first appearance in the Berea 
grit of Ohio until their extinction near the close of the Subcarboniferous, 
but those of Physonemus and Erismacanthus increase in complexity of or- 
namentation throughout the Mississippian series, ultimately displaying remark- 
able elaboration. An inspection of the forms illustrated in Plate XXII. of the 
sixth volume of the Illinois Palaeontology, and of the spines figured in the 
present contribution, will convince any one as to the correctness of these gener- 
alizations. 
The spines in the typical species and in others resembling it are much later- 
ally compressed, strongly arched or hook-shaped, with a broad base of inser- 
tion; the sides of the exserted portion are more or less ornamented with 
tuberculated longitudinal ridges, and small denticles are present upon the 
concave (posterior) border. This description applies to P. arcuatus M’Coy 
(the type species), P. attenuatus Davis, and P. hamatus (Agassiz), from the 
Carboniferous Limestone of Great Britain ; and to the American forms de- 
scribed as P. stellatus Newberry, and Drepanacanthus reversus St. John and 
Worthen. Another group of spines which may be referred provisionally to 
the same genus is typified by such forms as the so-called Drepanacanthus 
gemmatus Newb. and Worth., D. anceps Newb. and Worth., Xystracanthus 
acinactformis St. J. and Worth., Physonemus gigas Newb. and Worth., and the 
defences theoretically associated with the teeth of Polyrhizodus rossicus by A. 
Inostranzew} and O. Jaekel.? It is characteristic of the latter group of 
spines that they are forwardly curved, instead of backwardly, as in most 
Ichthyodorulites, a circumstance which appeared so anomalous to Newberry 
and Worthen as to warrant a generic separation from Physonemus. Transi- 
tional stages, however, showing the reversal of curvature from a posterior to 
an anterior direction, are to be observed in various species of Stethacanthus 
and Oracanthus, and for the present it seems best to extend the definition of 
Physonemus so as to include both groups. The two rod-like species from the 
Kinderhook limestone immediately to be described differ from all others in 
their more slender form and absence of ornamentation. They are undoubtedly 
to be interpreted as head-spines, a determination which is applicable to nearly 
all species of this genus. 
1 Travaux Soc. Nat. St. Petersb., Vol. XIX., 1888, pp. 1-18. 
2 Zeitschr. deutsch. geol. Ges., Vol. LI, 1899, p. 281, Fig. 5. 
