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, o. /. 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 



Scf^. 



op 



Fig. 919.— a, restoration of shield o£ Ceplialaspis lyelli, dorsal aspect ; B, diagi-am of ventral 

 aspect of shield of Cepbalaspis ; C, restoration of Cephalaspis murcliisoni ( De ronian). 

 03). opercular plate or pectoral fin ; or. orbit ; s. f. -p. sub-frontal plate. (From the Bi-it. Mvs. 

 Cat. o/FossU 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 3. — Antiarctia. 



This group contains five genera, of which Pterichthi/s (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 (;pa./.) of a very remarkable character, covered by strong scutes 



II 2 



