VI 
INTRODUCTION. 
elongated lobate fins flourish the earliest, all survivors above the 
Devonian having the lobe comparatively abbreviate. The large 
pair of gular plates so characteristic of the Crossopterygii is always 
wanting in the Chondrostei; but the lateral gulars are merely 
further subdivided to become the ordinary paired series of branehio- 
stegal rays, and in this connection it is noteworthy that both in 
Cheirolepis and many later Palaeoniscidae the subdivision is not 
complete anteriorly where a pair of very large plates persists. The 
ridge-scales on the upper caudal lobe of Chondrostei represent the 
superior rays of the caudal fin in Crossopterygii, as is indicated by 
the presence of the endoskeletal supports in Chondrosteus and the 
existing genera ; while it is not unlikely that the fulcra are modified 
enamelled anterior fin-rays such as occur, for example, in Osteo- 
lejpis. 
When the Chondrostei suddenly become dominant, as they do in 
the Lower Carboniferous, they already exhibit a remarkable series 
of modifications, which are enumerated in Part II. of this Catalogue. 
The family of Palaeoniscidae is represented chiefly by fishes with 
regular rhombic scales and distally-bifurcating fin-rays ; but there 
is one case in which the scales are cycloidal and deeply imbricating 
( Cryphiolepis), another in which the scales are absent except upon 
the caudal lobe (Phanerosteon), and a third in which the fin-rays 
never branch ( Holurus ). Some, moreover, have a large mouth with 
powerful conical teeth ; others, a small mouth with comparatively 
insignificant teeth. There is also a good deal of variety among the 
Platysomidae, which are deep-bodied fishes closely related to the 
Palaeoniscidae. 
As these fishes are traced upwards, they exhibit very little 
essential change. The upper caudal lobe never appears to atrophy 
in the least; the supports of the dorsal and anal fins never equal 
in number the appended dermal rays ; the infraclavieular plates 
always remain ; the ribs, if present, never ossify. The Platysomidae, 
indeed, become extinct in the Upper Permian, where, in addition 
to the normal genus Plcitysomus, there is a scaleless fish ( Dorypterus ) 
which seems to represent them. The large-mouthed and small¬ 
mouthed, rhombic-scaled and round-scaled, perhaps also the scaleless, 
Palaeoniscidae range as far upwards as the Permian; and then the 
family presents less variety in its representatives. All the Mesozoic 
genera are predaceous fishes with a large mouth and deeply over¬ 
lapping scales which are usually very thin; and the genus Cocco- 
lepis , which is the only known Palaeoniscid ranging above the Lias, 
