486 On the Fossil Fishes of the Carboniferous Limestone Series of Great Britavn 
and proportionately narrower ; the teeth on each side are shorter, more acuminate 
at the apex, and broader than the central one. 
The vertical sets of teeth (PI. LX1., figs. 16, 16a) are often in groups of five, occasion- 
ally fewer. The smallest tooth is nearest the base, and was the one first used ; 
the crown is about half the length of the tooth, which is half an inch. The second 
tooth rises beyond it ‘2 of an inch, and is °3 of an inch across the crown. A third 
and fourth tooth in succession have place before the newest and largest tooth is 
reached. The breadth of the convex crown of the latter, though not well pre- 
served, was ‘7 of an inch, and the total length from its apex to the basal extremity 
is 1-4 inch. From the base to the apex, the series of teeth present on the anterior 
face a uniform convexity, as represented in Pl, LXL, fig. 16a, whilst the opposite, 
or surface inside the mouth is concave, and formed simply of the crown and 
base of the longest and most prominent tooth, similar to the one represented 
in Pl. LXI., fig. 14. 
The crowns of the lower teeth are circular, or very slightly pointed ; the upper 
one, in use immediately prior to the decease of the fish, is pointed in the centre, 
and in some instances is very long compared with the breadth, as in the centre tooth. 
(Pl. LXL., figure 15.) It is probable, from the appearance of abrasion and wear of 
the enamel along the crest of the crown, and for a little distance down the convex 
face, that the lower teeth have each in turn occupied the primary position, and 
having served their purpose have been replaced by new teeth, which have grown 
or been raised up from the interior of the mouth ; the opposite to the method of 
increase generally observed in Elasmobranch fishes. The apex of the crown of 
each antecedent tooth is firmly pressed, and appears to become adherent, to the 
imbricated base of the convex crownof the following one. The smail teeth repre- 
sent an early stage of the fish’s existence, and each succeeding one, larger than the 
last, may indicate a more or less definite period, but certainly prove that as the 
fish increased in size its dentition, and the consequent extent of the gape of the 
mouth, also increased. 
Having considered the vertical arrangement, there remains the further question 
as to the lateral or transverse disposition of the teeth. Fortunately, a single spe- 
cimen occurs in the collection which throws considerable light on this part of the 
subject. It is a natural inference, from the arrangement of the three crowns, illus- 
trated in Pl. LXL, fig. 15, that the vertical series of teeth occupied a position side by 
side in a horizontal row ; and, judging from comparison with other and existing 
fishes, it is improbable that the entire series is represented by three teeth, but 
rather that it was more extensive, and consisted of at least seven teeth. This is 
rendered probable from the great resemblance existing between Petalorhynchus 
and the Janassa described by Count Minster, to be further alluded to. Whether 
seven was the number of teeth occupying each jaw at any given time or some other 
was the number may be doubtful. 
