On the Fossil Fishes of the Carboniferous Limestone Series of Great Britain, 487 
near the basal extremity may have been separated one from the other, so as to 
give a more expanded disposition to that portion of the teeth; but, that such was 
the case is at any rate doubtful, because in the young fish, when only the smallest 
or earliest set of teeth existed, they would extend in close contiguity round the jaw. 
On the advent of the second set, their larger and broader crowns extended beyond, 
and above the first, and so enlarged the surface and diameter of the gape. In the 
same way, the further additions of the third, fourth and fifth rows constantly 
increased the size of the mouth with the growth of the fish ; and it does not seem 
probable that the earliest teeth changed from their original position relative to each 
other. It is also worthy of remark, that the majority of the vertical sets exhibit a 
more or less bilateral arrangement, the lower teeth inclining to the right or left, 
and having the appearance which would be presented if they had been placed at 
the side of a central row, being inclined at an angle fitting them to occupy such a 
position. 
The teeth already described are in all probability the primary teeth of the lower 
jaw. The primary or largest tooth of the central vertical series and the one in use 
was longest, and proportionately more pointed than those on each side of it. It is 
probable that the central primary tooth of the upper jaw was longer and narrower 
than those of thelowerjaw,and that thethree teeth represented in Pl. LXL, fig. 15, are 
those of the upper jaw. In this example the length of the crown of the central tooth is 
greater than one and a half times its breadth. It is pyramidally cone-shaped, and 
converges to a bluntly-pointed apex. There are imbricating folds across the base 
of the crown extending in parallel, slightly-curved lines, but not with the peculiar 
sigmoidal curvature of the lateral teeth. The base is proportionately very long 
and strong—almost quadrate in section —composed of coarsely-striated osseous 
tissue. The second, third,’ fourth, and fifth teeth succeed each other im diminish- 
ing sequence, the smallest tooth being nearest the base. Their general character is 
similar: the concave posterior surface of the crown of the second tooth rests on 
the convex anterior surface of the one preceding it, and so on, The apex or cut- 
ting edge of the crown in the secondary and succeeding teeth is, in almost all 
cases, circular, and without the pointed apex of the primary teeth. This appears, 
in all probability, to be due to those teeth having previously served as primaries, 
and become worn during attrition, the most recent and largest teeth being those 
in use. In each case the apex of one tooth rests on the imbricated folds of the 
anterior coronal ridge of the preceding one, and must serve as a substantial support 
to it. The whole series form a strongly convex mass. So close is the contact 
between the lower basal portion of the series of teeth that their planes of division 
appear to have become obsolete, and the whole anchylosed into one solid body. It is 
difficult to say whether this be really the case; but where a set of the teeth have 
become fractured, it is almost impossible to trace any division between them. 
Whilst searching for affinities or relationship of this peculiar arrangement of 
TRANS, ROY. DUB. SOC., N.S,, VOL. & 4F 
