GANOIDEI. 331 



They have generally been placed amongst the Ganoids ; but 

 Professor Huxley has pointed out that they present, many of 

 them, features by which they approximate closely to the Silu- 

 roids amongst the Teleosteans. The more important genera 

 included in this sub-order are Cephalaspis, Pteraspis, Coccosteus, 

 and Pterichthys. 



Cephalaspis (fig. 292) is the type of the family of the Cephal- 

 aspida, and is readily recognised by the fact that the cephalic 

 shield has its posterior angles produced into long " cornua," 



Fig. 292. Cephalaspis Lyetlii. (After Page.) Old Red Sandstone. 



giving it the shape of a " saddler's knife." Besides these la- 

 teral cornua, there is a " posterior cornu " or spine, formed by 

 a prolongation backwards of the hinder margin of the shield in 

 the middle line. The orbits are approximated, and are placed 

 nearly in the centre of the cephalic shield. No jaws or teeth 

 are known, and the mouth was probably soft, and adapted for 

 suction. The head-shield exhibits vascular canals, and shows 

 very distinct bone-cells when examined in thin sections under 

 the microscope. The body is covered with ganoid scales, and 

 there is a well-marked dorsal fin. Pectoral fins have also been 

 described, and the tail is clothed with a heterocercal fin. In 

 the nearly allied Auchenaspis, the structure is very similar to 

 the above, but there is no spine or " posterior cornu/' and 

 there is instead a neck-plate formed by an extension backwards 

 from the cephalic shield. The Cephalaspidce are mainly found 

 in the Old Red Sandstone, the commonest species being C. 

 Lyellii. Other species are found in the " passage-beds " 

 between the Silurian and Old Red, and the genus is not 

 wholly unrepresented in the Upper Silurian deposits. 



In the genus Pteraspis (fig. 293) the head is defended, as in 

 Cephalaspis, by a shield or buckler, which is composed of seve- 

 ral pieces firmly anchylosed. The shield consists of a central 

 disc, the lateral angles of which are produced into short cornua, 



