EASTMAN: CARBONIFEROUS FISHES FROM THE CENTRAL WEST. 213. 
the condition observed in certain Mesozoic Teleosts, such as Coccodus from the 
Lebanon Cretaceous, in which very similar head-spines are developed, and 
oriented in the same way, one on either side of the head. Appearances sug- 
gest that the long and stout anterior branch in the present form was for the 
most part buried in the integument, only the double row of robust conical 
denticles protruding, but the posterior spine was probably entirely exposed. 
The typical example of this species, shown in Text-figure 13 and now deposited 
in the Museum of Comparative Zoology, exhibits a total length of no less than 
21.5 cm. 
Formation and Locality. — St. Louis Limestone; Missouri. 
STETHACANTHUS Newserry. 
Definition (emend.). — Spines broad, much laterally compressed, triangular 
or falcate in outline, deeply inserted. The elongated basal portion thin-walled 
and occupied by a very large internal cavity which extends upward nearly to the 
apex; exserted portion terminated at one end bya strong, often tumid shoulder, 
and rising at the other by a gradual curve into an acuminate summit. Sur- 
face of spine having a rough, fibrous appearance and marked in the basal 
portion by coarse vascular impressions. Internal structure distinctly fibrous, 
owing to the fan-shaped radiation of numerous fine tubules from the apex 
toward the basal portion. Apex usually inclined posteriorly (7. ¢., away from 
the tumid “shoulder’’), but sometimes erect, or even slightly inclined 
forwards. 
The peculiar class of dermal structures which are recognized under the name 
of Stethacanthus display quite uniform characters throughout their range from 
the Waverly to near the summit of the Mississippian series. They were re- 
garded as pectoral fin-spines by Newberry,! who was under the mistaken im- 
pression that they were not bilaterally symmetrical, and was further misled by 
the fortuitous association on the same slab of a spine of S. tumidus with fin- 
rays of an Actinopterygian fish. There is no reason for supposing that they 
were situated elsewhere than in the median line of the body, either along the 
back or at the base of the head. 
On considering the probable relationships of Stethacanthus, we are struck 
immediately with its resemblance to Physonemus, especially such forms as P. 
gigas, nor can a certain similarity be denied to the remarkable Kinderhook 
spine described by Newberry as Oracanthus vetustus, to which reference has 
already been made. In the latter form the base is much produced in an an- 
terior direction, forming a most efficient anchorage in the soft parts for the 
exserted portion, and it is noteworthy that the tip of the exserted portion is 
slightly curved forwards. The same condition of things is developed toa 
somewhat lesser extent in Physonemus gigas, in which there is even an incipi- 
ent “shoulder” at the base of the concave (anterior) margin. A more 
1 Monogr. U.S. Geol. Surv., Vol. XVI., 1889, p. 198. 
