152 HISTORICAL PALAEONTOLOGY. 



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, b) 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 Fteraspis 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 Ptcrichthys (fig. 104). In the former of these the 



Fig. 103. Cephalaspis Lyellii. 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 Ptcrichthys 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 



