On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 338 
The genus Ctenacanthus is confined to the strata below the Coal Measures, just as 
the genus Hybodus has only been found in the rocks above. The two genera are 
very similar in form, and the fishes which were possessed of the spines were ‘evi- 
dently closely related in structure and habits. Prof. Agassiz (‘‘ Poiss. Foss.,” Vol. 
III., p. 171) considered that the spines of Ctenacanthus and the teeth of Psammodus 
might have belonged to the same fish, but the discovery of the teeth of Hybodus in 
close relationship to the position of the spines, has proved beyond doubt that the 
teeth of that genus bear a close relationship to the pointed cutting teeth of the 
sharks of the existing species, having a central prominent pointed cone with smaller 
lateral ones rising from a broad expanded base. The occurrence in considerable abun- 
dance of the teeth of Cladodus in the Carboniferous rocks, similar in form to those 
of the known Hybodus, renders the probability great, that Ctenacanthus was pro- 
vided with teeth possessing similar characters to those of Hybodus and that the 
spine of Ctenacanthus and the teeth of Cladodus may have been from the same 
fish, though no positive proof exists of the relationship. 
The Agassizian conception of the genus Ctenacanthus has been enlarged by various 
authors so as to include a number of specimens, like Ctenacanthus (?) distans, M‘Coy, 
which it is very probable pertained to quite a different type of fish. The inclusion 
of such species has been also made by some American paleontologists, as for instance, 
C. gracillimus (“Geological Survey of Iilinois,” Vol. IL, p. 126, pl. xiii., fig. 3), 
C. burlingtonensis and C. keokuk, St. J. and W. (‘“Illinois,” Vol. VI., pp. 426, 427, 
pl. xv., figs. 7, 8), and others. These specimens differ in no respect from some which 
have been described as Leptacanthus, Ag. (compare C. gracillimus with L. occidentalis, 
op. cit. pl. xil., fig. 2) and Acondylacanthus, St. J. and W., whilst they present 
ereat divergence from the Ctenacanthoid type. A chief characteristic of the 
latter is the large open posterior groove or pulp cavity extending the whole length 
of the osseous base and in a few instances still further up the spine. This feature 
appears tobeentirely absent in the long graceful spines indicated above. In the large 
series from the Armagh limestone there is no evidence of such a cavity opening 
posteriorly, the internal cavity is terminal and is open only at the end of the base 
of the spine. After searching through the descriptions of American paleontologists 
a similar result is obtained. In nearly all cases the basal end is fractured or 
wanting. This difference may possibly be of even greater than generic importance, 
and may indicate affinities rather with the Batoidei than the Selachoidei. In the 
following descriptions it is proposed to remove the variety of spines indicated trom 
the genus Ctenacanthus, and, for reasons hereafter adduced, to place them with the 
so-called Carboniferous Leptacanths in the genus Acondylacanthus of Messrs. St. John 
and Worthen ; retaining only such species in the genus Ctenacanthus as will fall in 
with the description of them indicated by Professor Agassiz. 
