EASTMAN: CARBOXIFEROUS 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 tlie present form was for tlie 

 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 Newberry. 



Definition (enuiul.). — 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 by a 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 {i. c, 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. 

 gifjas, 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 to a 

 somewhat lesser extent in Physoiiemus 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. 



