GANOIDEI. 



393 



the pectoral fins is the peculiar characteristic of the Pterichthys. 

 These were in the form of two long curved spines, something 

 like wings, covered by finely-tuberculated ganoid plates. From 

 their form, they cannot have been of much use in swimming ; 

 but they probably, as suggested by Owen, enabled the fish to 

 shuffle along the sandy bottom of the sea, if left dry at low 

 water. 



b. Pteraspis. — In most respects this genus was not unlike 

 Pterichthys, but it did not possess the peculiar pectoral fins of 

 the latter. One species of Pteraspis has been found in the 

 Upper Silurian Rocks (Ludlow), and is as yet one of the earliest 

 known indications of the appearance of the great sub-kingdom 

 Vertebrata upon the globe. 



c. Cephalaspis (fig. 147). This, again, is not unlike Pterich- 

 thys in many respects. The cephalic buckler, however, has 



Fig. 147. — Cephalaspis Lyellii. 



its posterior angles produced backwards, so as to give it the 

 shape of a " saddler's knife," whilst the pectoral limbs have not 

 the form of spines. 



d. Coccosteus (fig. 148). — This is another charaqteristic 



Fig. 148. — T. Coccosteus decipiens; 2. Pterichthys Milleri. 



genus of the Old Red Sandstone. In this genus, as in the 

 preceding, there is a cephalic buckler, the plates of which aie 



