XIII 



PHYLUM CHORDATA 



263 



rescnililaiice to tlic ft'|)li;ili(; shield of Liimilus of of a Trilobite, being gently 

 ciirvc'cl above, proiluceil beliind into spines, eontinnetl venti'ally into a sub-fioiital 

 plate (B, s./. p.), ami liaviiig a pair of orbits (A, or) for the eyes near the middle 

 of tile dorsal snrfaee. Behind tlie siiield, towards the ventral surfaee, is a jjlate 

 whieli perhaps supported the opeieuhun ((.', ojj.), but may represent the pectoral 



o/* 



s^^ 



or- 



op 



Fk;. '.ai).— a, restoration of shit-kl of Cephalaspis lyelli, dorsal asjicL-t : B, diagram of ventral 

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

 o-p- opercular plate or pectoral fin ; or. orbit ; s. f. p. sub-frontal plate. (From the Brit. Mvs. 

 Cat. of Fossil Fishes.) 



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

 bone. The posterior portion of the body is covered I)}' deep, narrow scales ; 

 there is a single dorsal tin and a heteroeercal caudal. 



Order 3. — Antiakcha. 



This group contains five genera, of wiiich Ptcrirhtltij-s (Fig. 920) may Ije taken as 

 an example. It presents abroad and high anterior region, covered hy articulated 

 plates which have the structure of bone and are covered by a laj'er 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 Ijctween them is a plate pitted 

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

 pectoral tins (^ycf. /.) of a very remarkable character, covered by strong scutes 



R 2 



