INTRODUCTION. 
XIII 
necessity, and not a character of taxonomic importance. The 
absence of a maxillary dentition is not remarkable, considering its 
feebleness in the species of Lepiclotus with most specialized grinding- 
teeth. 
Secondly, there is the question of the mode of growth of the 
dentition. There may sometimes be vertical successional teeth, but 
the present writer has never observed satisfactory evidence of them ; 
even if occurring, they must thus he extremely unusual. The contrast 
with Lepidotus is indeed very marked ; hut the loss of successional 
teeth in the higher Yertehrata (e.g. in elephants and kangaroos) is 
a sign of great specialization, and it is not improbably the same 
among these fishes. So far as the dentition is concerned, Lepidotus 
need not be more widely separated from Mesodon than is the extinct 
Dinotherium from the modern elephant. 
Thirdly, the enormous development of the preoperculum and the 
concomitant reduction of the operculum are again paralleled in 
distantly-related fishes, which possess similarly powerful, short jaws 
in a very forward position. The existing genera Tetrodon and 
Lampris may be cited as illustrations. Once more it is thus evi¬ 
dent that we are concerned with a physiological correlation of no 
fundamental significance, and one which is the mere outcome of 
specializations towards which there is already a tendency in the 
Semionotidse. 
Finally, the anomalous character of the cranial roof is at first 
more difficult to understand. It will, however, he observed that 
there is a most striking resemblance between this roof as known 
in Mesturus (p. 192, fig. 30) and that of the modern sturgeon 
(. Acipenser ). Moreover, the disposition of the sutures is evidently 
as capricious as that in Acipenser ; for not only do the parietal 
plates lack bilateral symmetry in the specimen figured, hut there 
is also considerable difference in the arrangement of the median 
plates in a second specimen in the Leeds Collection, which is 
otherwise very similar, blow, it has already been shown (p. viii) 
that Acipenser is merely the surviving terminal form of a long 
series of Chondrostean fishes, which in the Liassic period still re¬ 
tained a normal cranial roof. Such having been demonstrated 
among Chondrostei, it would not be surprising to find a similar case 
of development or degeneracy among Protospondyli. In the present 
state of knowledge it appears likely that this parallel case is to be 
recognized among the Pycnodonts. 
The result of these considerations is, therefore, that the Pycno- 
