GANOIDEI. 369 



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. 139). This, again, is not unlike Pterich- 

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

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

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

 the form of spines. 



" Fig. 139. Cephalaspis Lyellii. 



d. Coccosteus (fig. 140). This is another characteristic 

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

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



Fig. 140. i. Coccosteus decipiens; 2. Pterichthys Milleri. 



covered with small hemispherical tubercles. The notochord 

 was persistent, but the neural and haemal spines, and the rays 

 of the dorsal and ventral fins, are well ossified. A large heter- 

 ocercal tail-fin was doubtless present as well. 



VOL. II. 2 A 



