152 HISTORICAL PALZONTOLOGY. 
more or less remotely, by a few Devonian fishes—such as As- 
terosteus ; and the great Macropetalichthys of the Corniferous 
limestone of North America is believed by Newberry to belong 
to this group. In this fish (fig. 102, 6) the skull was of large 
size, its outer surface being covered with a tuberculated en- 
amel; and, as in the existing Sturgeons, the mouth seems to 
have been wholly destitute of teeth. Somewhat allied, also, to 
the Sturgeons, is a singular group of armoured fishes, which is 
highly characteristic of the Devonian of Britain and Europe, 
and less so of that of America. In these curious forms the 
head and front extremity of the body were protected by a 
buckler composed of large enamelled plates, more or less 
firmly united to one another ; whilst the hinder end of the body 
was naked, or was protected with small scales. Some forms of 
this group—such as Freraspis and Coccosteus—date from the 
Upper Silurian ; but they attain their maximum in the Devo- 
nian, and none of them are known to pass upwards into the 
overlying Carboniferous rocks. Amongst the most character- 
istic forms of this group may be mentioned Cephalaspis (fig. 
103) and Prerichthys (fig. 104). In the former of these the 
Fig. 103.—Cephalaspis Lyetliz. Old Red Sandstone, Scotland. (After Page.) 
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 Prerichthys—which, like the preced- 
ing, was first brought to light by the labours of Hugh Miller— 
the whole of the head and the front part of the body were de- 
fended 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 
