ON SOME ARMOURED FISHES. 
19 
The genus Chondrosteus carries this type back as far as the 
Lias period. 
In the Placodermi, represented by Dinichthys , Goccosteus , 
and Pterichthys , we have a somewhat further advance in the 
ossification of the neural and haemal spines and of the appen- 
dages of the rays of a u dorsal,” “ anal,” and caudal fin, the 
head and anterior portion of the trunk being, at the same time, 
encased in dermal bony plates. 
The cartilaginous heterocercal-tailed sharks,* * * § with partially 
ossified endoskeleton, and a covering of skin, toughened 
with shagreen, tubercles, and spines, belong to an ancient 
tribe of fishes whose progenitors probably date back to Upper 
Silurian times. 
The bony pikef ( Lepidosteus ) of the American rivers, and 
the Polypterus% of the Nile and Senegal, are the best living 
exemplifications of the old bony-scaled ganoids — fishes clad in a 
complete suit of enamelled armour, which, beginning with 
Osteolepis in the Devonian, attained their maximum development 
in Mesozoic times. The vertebral column in Lepidosteus is more 
perfectly ossified than in any other fish, the vertebrae are re- 
markable as being opisthocoelian in type (hollow behind, convex 
in front). 
The thin-scaled homocercal Caturidae (see woodcut, Fig. 15), 
with well ossified head-plates, neural and haemal appendages, 
and fin-rays, but with persistent notochord, seem to mark the 
incoming of our modern form of fishes. § 
In the Cretaceous formation the Teleostian, or well ossified 
bony fishes, are numerous, and also fishes with flexible 
64 cycloid ” and “ ctenoid ” scales. The order Teleostei includes 
the majority of existing fishes, as well as those which made 
their appearance in Tertiary times. 
Concerning the last and highest group of which we have 
spoken — the Protopteri of Owen, or Dipnoi of Muller— we 
venture to think a strong case might be established for remov- 
ing these anomalous forms across the border-line of the class 
Fishes, and placing them with the Amphibia. 
There is no tendency, either in Lepidosiren , or in Geratodus , 
to produce those unpaired fins (“ dorsal ” and 44 anal ”), or the 
* Plagiostomi (Muller) = Elasmobranchii (Bonaparte). 
t Lepicloganoidei, sulb-ord. Lepidosteidce (bony-scaled). 
X Lepicloganoidei, sub-ord. Crossopterygidce (fringe-finned). 
§ “ Cycloid ” fish scales were noticed by Mr. C. B. Kose,F.G.S., in the Kim- 
meridge clay, Ely, and in Norfolk ; others have been noticed by Mr. W. 
Davies, F.G.S., in the British Museum, from the same locality. See u Geol. 
Mag. 1864, Yol. i. pp. 92-94. 
