Amongst the most characteristic
forms of this group may be mentioned _Cephalaspis_ (fig. 103) and
_Pterichthys_ (fig. 104). In the former of these the head-shield is
of a crescentic shape, having its hinder angles produced backwards
into long "horns," giving it the shape of a "saddler's knife."
No teeth have been discovered; but the body was covered with
small ganoid scales, and there was an unsymmetrical tail-fin.
In _Pterichthys_--which, like the preceding, was first brought
to light by the labours of Hugh Miller--the whole of the head
and the front part of the body were defended by a buckler of
firmly-united enamelled plates, whilst the rest of the body was
covered with small scales. The form of the "pectoral fins" was
quite unique--these having the shape of two long, curved spines,
somewhat like wings, covered by finely-tuberculated ganoid plates.
All the preceding forms of this group are of small size; but
few fishes, living or extinct, could rival the proportions of
the great _Dinichthys_, referred to this family by Newberry.
In this huge fish (fig. 102, a) the head alone is over three
feet in length, and the body is supposed to have been twenty-five
or thirty feet long. The head was protected by a massive cuirass
of bony plates firmly articulated together, but the hinder end
of the body seems to have been simply enveloped in a leathery
skin.
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