202 PALEOZOIC FISHES OF NORTH AMERICA. 
considerably like that of Ct. Harrisoni,’ St. J. & W., but is much more 
crowded and the form of the spine is narrower and straighter. 
Formation and locality: Saint Louis limestone; Saint Louis, Mo. Col- 
lected by Prof. A. Litton, who has contributed many new things from this 
geologic horizon and locality, and to whom it is dedicated. 
CTENACANTHUS CYLINDICUS, Nn. Sp- 
Plate XXVI, Fig. 1. 
Spines straight or slightly curved, from eight to twelve inches in length, 
broadly arched or crescent-shaped below, nearly cylindrical from the mid- 
dle upward: medullary cavity large, open posteriorly above the middle ; 
posterior face traversed by a very strong arched ridge; line separating 
plain from ornamented surfaces very oblique; all exposed surface occupied 
with enameled ridges, which are coarsest along the anterior face, where they 
frequently fork; near the posterior margin they are finer and more continu- 
ous; the pectination, which is general, is almost entirely confined to the 
sides of the ridges, the summit of each ridge being nearly smooth, the sides 
strongly denticulated; near the posterior margin the narrower ridges are 
slightly knobbed or beaded, as in many other species. 
The most striking peculiarities of this strongly marked spine is its 
elongated and cylindrical form, and the coarse, dichotomous ridges, smooth 
on the top and denticulated on the sides. The specimens I have are incom- 
plete, and show no denticulation of the posterior surface. This was probably 
present, however, near the summit. Another specimen than that which has 
formed the basis of the above description indicates a shorter and straighter 
spine, in which the ornamentation, though of the same peculiar character, 
is more sparse and open, and I have suspected that this was a posterior 
dorsal spine. Numerous dermal tubercles which occur with these spines 
indicate that the shark which bore them was protected by a coarse shagreen 
of which the individual tubercles were ornamented with concentric lines. 
Formation and locality: Keokuk shale; Casey County, Ky. Collected 
by Mr. William M. Linney. 
! Geol. Survey Illinois, vol. 7, 1883, p. 236, pl. 23, fig. 1. 
