332 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 
instance by a pavement-like surface of crushing teeth. As will be seen, however, 
there appears to be a wide difference in the process of growth as well as form, and 
when it is remembered that magnificent, almost perfect, palates of Agassizodus have 
been found in the Carboniferous Measures of {llinois, described by Messrs. St. John 
and Worthen (“Geol. Survey of Illinois,” Vol. VI., p. 311), and comprising a variety 
of teeth closely resembling those of Orodus, which are arranged almost exactly as 
are those of the existing Cestracion, little doubt can be entertained that there is in 
the Cochliodonts a group of fishes whose representatives have long ceased to exist, 
and which must stand apart and alone in ichthyic development. 
Besides the Psammodontide, represented by the well-known Psammodus, the 
Copodontidee comprise a large assemblage of more or less flat pavement-like teeth 
from the Armagh limestone. This large assemblage, the result of many years most 
painstaking collection, is quite unique. Inno other locality does it appear that teeth 
of similar import have been discovered. They have remained in the collection of Lord 
Enniskillen, undescribed to the present time. Professor Agassiz, during a visit to 
Florence Court nearly twenty-five years ago, carefully examined the collection and 
distributed the specimens amongst a number of new genera and species, to which 
he appended names in manuscript. These have been retained in the following 
pages in every possible instance. 
Some light is thrown on the dentition of Psammodus by specimens in the Bristol 
Museum, and it appears probable that the idea given by M. de Koninck (“ Faune 
du Cale. Carbon. de la Belgique,’ p. 42) of the dentition of Psammodus may 
prove the correct one. 
A. Selachoidet. 
Family —Hybodontide, Agass. 
«Two dorsal fins, each with a serrated spine. Teeth rounded, longitudinally 
striated, with one larger and from two to four smaller lateral cusps. Skin covered 
with shagreen.”—(Giinther.) 
Genus——Ctenacanthus, Agass. 
Ctenacanthus—Agassiz, L., 1833. ‘ Recher. Poiss. Foss.,” Vol. III., p. 10. 
M‘Coy, F., 1855. “ Brit. Paleeoz. Foss.,” p. 624. 
oP) 
‘‘Fin-spines, of moderate or large size, compressed, gradually tapering, moderately 
arched backwards ; anterior face narrow, rounded ; posterior face concave, with a 
moderate cavity, the lateral edges bordered by two rows of curved denticles 
inclined downwards. Surface marked with strong, longitudinal ridges and furrows, 
pectinated by transverse scales or tubercles. The concealed base of moderate size, 
rapidly tapering, finely striated.” —(M‘Coy.) 
