XX 
INTRODUCTION. 
hitherto been discovered in the jaw. It is thus very probable that 
the Pholidophoridae are the earliest known family of Isospondyli; 
and if more conclusive specimens eventually confirm this inference, 
the suborder is proved to date back at least to the Upper Trias. 
In any case it is remarkable that Lejptolepis and its immediate 
allies were a comparatively insignificant race until the Cretaceous 
period, when the Isospondyli and higher suborders became domi¬ 
nant ; and they form an interesting example of the long persistence 
of a newly-acquired character before its possession becomes a factor 
of real importance in favouring the supremacy of the type. 
The Pholidophoridse, as defined in the Catalogue, are the only 
fishes with articulated rhombic scales which can be reasonably 
claimed as Isospondyli; and even the peg-and-socket articulation 
is inconstant if the genus Archceomcene (p. 488) is correctly associated 
with them. The most generalized member of the family, Pholido- 
phorus itself, ranges practically unchanged from the Upper Trias, 
throughout the Jurassic, as far as the Purbeckian. An apparently 
allied genus ( Thorcicopterus ), with excessively enlarged pectoral fins, 
is restricted to the Upper Trias. The Triassic Pholidopleurus , as 
already mentioned, is very doubtfully placed here ; but two other 
genera with much-deepened flank-scales, namely, Peltopleurus of the 
Upper Trias, and Pleuropliolis of the Upper Jurassic and Pur¬ 
beckian, may be ascribed to the Pholidophoridse with less hesitation: 
and there is possibly an almost scaleless representative of the family 
(Ceramurus) in the Purbeckian. One unique feature is also worthy 
of special note in Pleuropliolis, namely, that the canal of the 
“ lateral line ” is deflected immediately behind the pectoral arch 
and extends not along the deepest flank-scales but along the 
adjoining series below. 
OUc/opleuridce. 
The few Upper Jurassic and Cretaceous fishes included in this 
family bear much superficial resemblance to the Amiidse. All of 
them, however, exhibit completely ossified vertebral centra, never 
any alternating pleurocentral and hypocentral discs even in the 
caudal region; while the mandible appears to be destitute of 
splenial and coronoid elements, and thus confirms the suspicion 
suggested by the first character, that these fishes must be referred 
to a higher suborder than that of the Protospondyli. The fins, like 
those of the Pholidophoridse, are fringed with fulcra ; and it seems 
most reasonable in the present state of knowledge to place the 
