ORDER GANOIDEI. 



961 



founded. Holaspis, from the Lower Devonian of North Wales, is 

 distinguished by the dorsal shield, consisting of only a single 

 element. Figure 888 is an attempt to restore the form of the type 



Family Cephalaspidim:. — The Cephalaspids are regarded by 

 some writers as belonging to the Pteraspididce, while by Professor 

 Zittel they are placed in a distinct suborder. The dorsal cephalic 

 shield (fig. 889) consists of one or two pieces, and usually has more 

 or less developed lateral cornua, with a regularly curved and flat 

 lower border — the eyes being situated near the middle ; in structure 

 it resembles true bone. The body is covered with bony plates, and 

 the caudal fin is heterocercal. The typical genus Cephalaspis (figs. 



Fie. 88 



y. — Left lateral view of Cephalaspis Lyelli ' ; from the Devonian of Scotland. 

 About one-third natural size. (After Lankester.) 



590) has the cephalic shield single, and may be divided into 

 the Eucephalaspidine and Hemicephalaspidine groups, according to 

 the degree of development of the cornua. It occurs in the Si- 

 lurian and the Devonian of Britain, and has also been obtained 



Fig. 890. — Smaller views of Cephalaspis Lyelli. (After Page.) 



from the Silurian of Bohemia and the Devonian of Canada. Xen- 

 aspis, from the same English formations, is distinguished by the 

 presence of one or more quadrangular dorsal plates behind the 

 head ; while Auche?iaspis, of the European Silurian, has the cephalic 

 shield transversely divided. Didymaspis, from the same formation, 

 agrees with the latter in its double cephalic shield, which is, how- 



