On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain. 415 
mandible, a part of which extended inwards from the jaws and formed a covering or 
base for the palates which may or may not have had other teeth attached. None 
of the present specimens exhibit any trace of other teeth having keen so attached, 
and though examples have been discovered by American palzeontologists in which the 
teeth of Cochliodus and a species of Helodus have been found associated in such a 
manner as to lead to the inference that they belonged to the same fish, this evi- 
dence has not hitherto been confirmed by any British species. 
As already stated, Dr. Giinther (‘Study of Fishes,’ p. 329), places the 
Cochliodonts in his family Cestraciontide, along with Ctenoptychius from the 
Devonian; Psammodus, Chomatodus, Petrodus, Polyrhizodus, &c., from the Carboni- 
ferous series; and Strophodus, Acrodus, Thecodus and Ptychodus, from the Trias 
and Chalk. | 
Having in the foregoing brief resumé glanced at the classification of the group 
of fishes represented by the fossil Cochliodus adopted by the several authors enu- 
merated, it is proposed to describe, in such detail as may be necessary, the dentition 
of the members of the group which it is possible satisfactorily to interpret. The 
elucidation of the subject is, however, attended with many difficulties, and it is only 
tentatively that the attempt is now made: the complete reconstruction, on a 
thoroughly sound basis, of the dentition of this large and important group of fishes 
must depend in a very great measure on the acquisition of more perfect specimens. 
Much information can be gathered from the study of the unique collection of this 
group collected by Lord Enniskillen, and considerable light has been thrown on the 
genus Cochliodus by some magnificent specimens gathered by American paleeonto- 
logists from the Carboniferous Measures so largely developed in Illinois and Ohio, 
but the complete and perfect dentition of any single fish has not yet been discovered 
in a state sufficiently well and regularly preserved to warrant the statement that it 
is perfectly known and consequently comparable or otherwise with that of existing 
fishes. 
The teeth of the group of fish remains, for which Prof. Owen has proposed the 
name Cochliodontide, differ much in the details of their form and arrangement, 
but they possess the chief character in common, which is the distinguishing feature 
of the group, viz :—that they grow or increase in size, not as in most fishes, by the 
old and worn teeth being replaced from behind or below, but by continuous or 
repeated additions to the inner or posterior margin of the surface of the tooth, so 
that the same tooth is always increasing in size with the growth of the fish, and 
assumes an inrolled or convoluted form. An analogue of this peculiar growth is 
seen in that of the testaceous covering of the mollusca, which, though it assumes a 
wonderful variety of forms increases in size by the addition of repeated layers to 
the open margin of the shell. A transverse section of the teeth of Cochliodus, 
Streblodus, Deltodus or Deltoptychius, exhibits a more or less helecine involution, 
whilst those of Psephodus, Peecilodus or Xystrodus, though flatter, increase by the 
radial expansion of the triturating surface. 
TRANS, ROY, DUB, SOC., N.S., VOL, I. cau 
