XTII 



PHYLUM CHORDATA 



263 



resemblance to the cephalic shield of Limulus or of a Trilobite, being gently 

 curved above, produced behind into spines, continued ventrally into a sub-frontal 

 plate (B, s.f. p.), and having a pair of orbits (A, or) for the eyes near the middle 

 of the dorsal surface. Behind the shield, towards the ventral surface, is a plate 

 which perhaps supported the operculum (C, op.), but may represent the pectoral 



or 



FIG. 919. A, restoration of shield of Cephalaspis lyelli, dorsal aspect ; B, diagram of ventral 

 aspect of shield of Cephalaspis ; C, restoration of Cephalaspis murchisoni (Devonian). 

 op. opercular plate or pectoral fin ; or. orbit ; s. /. p. sub-frontal plate. (From the Brit. Mv.s. 

 Cat. of Fossil Fishes.) 



fin. The scutes contain some lacunae, and therefore approach in structure to 

 bone. The posterior portion of the body is covered by deep, narrow scales ; 

 there is a single dorsal fin and a heterocercal caudal. 



ORDER 



-ANTIARCHA. 



This group contains five genera, of which Plerichthys (Fig. 920) may be taken as 

 an example. It presents a broad and high anterior region, covered by articulated 

 plates which have the structure of bone and are covered by a layer of enamel, 

 and a caudal region covered by rounded or hexagonal scales. The orbits are 

 placed close together on the top of the head, and between them is a plate pitted 

 on its inner surface, apparently for the pineal body. There is a pair of large 

 pectoral fins (pet. f.)oi a very remarkable character, covered by strong scutes 



R 2 



