
850 THE AMERICAN NATURALIST. (VoL. XXXVI. 
The Physonemus group of spines has been theoretically associ- 
ated by Jaekel with the teeth of petalodonts, but this conjecture 
has not been corroborated as yet by any direct evidence. 
Very interesting stages of modification are displayed by the 
group of Physonemus-like spines throughout their existence in 
the Lower Carboniferous. The earliest and most primitive 
forms of the typical genus are found in the Kinderhook, in 
accompaniment with small forms of Stethacanthus. The two 
or more species of Kinderhook Physonemi that are known are 
of small size, attenuated, and quite destitute of surface ornamen- 
tation. Erismacanthus is also represented in the Kinderhook 
by two small, comparatively unornamented species, and it is 
noteworthy that the branched condition is here rudimentary. 
The Burlington species of Physonemus and Stethacanthus 
display a marked increase in size, but the ornamentation is 
feeble, and remains so throughout the stage represented by the 
Keokuk Limestone. Stethacanthus seems to have attained its 
maximum size in the Keokuk, as Physonemus did in the Bur- 
lington (P. gigas N. and W.), a considerable falling off in this 
respect being true of both genera in the St. Louis division. 
The spines of Stethacanthus remain unornamented from their 
first appearance in the Berea Grit of Ohio until their extinction 
in the St. Louis Limestone, but those of Physonemus and Erisma- 
canthus increase in complexity of ornamentation throughout the 
Mississippian series, ultimately displaying great elaboration. 
An inspection of the forms illustrated in Plate XXII of the 
sixth volume of the 7//inois Paleontology, or of the large spine 
immediately to be described, will, we think, satisfy any one as 
to the correctness of this latter statement and of the above 
generalizations. 
Erismacanthus formosus sp. nov. (Fig. 1). 
DEFINITION. — A very large species, the spines attaining a total length 
of at least 22 cm. Anterior branch stout, much produced, gently arched, 
one side more or less flattened and provided with a double row of obtusely 
conical denticles, the other smooth and convex, appearing as if imbedded. 
Posterior spine considerably arched, prominently keeled, laterally com- 
: and bearing a double series of closely set denticles along the distal 
; half of the posterior margin. The posterior spine is relatively wide in its 
