ORDERS OF FISHES. 147 



closely to the Siluroids amongst the Teleosteans. The more 

 important genera included in this sub-order are CephalasiDis, 

 Fteraspis, Coccostcus, and Ptcrichtliys. 



Ceplialaspis (fig. 512) is the type of the family of the 

 CephalaspidcG, and is readily recognised by the fact that the 

 cephalic shield has its posterior angles produced into long 



Kg. 512. — Ceplialaspls LyelUL (After Page.) Old Red Sandstone. 



" cornua," giving it the shape of a " saddler's knife." Besides 

 these lateral 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 Eed Sandstone, the commonest species being C. Lydlii. 

 Other species are found in the " passage-beds " between the 

 Silurian and Old Eed, and the genus is not wholly unrepre- 

 sented in the Upper Silurian deposits. The genus occurs 

 not only in Europe, but also in the Devonian of North 

 America. 



