﻿CEPHAXA.SPID.E. 187 



oblong portions, the areas immediately within this are at least as 

 long as broad, while those forming the greater part of the " flap " are 

 considerably broader than long, and are suggestive of a certain 

 amount of flexibility in the original structure. The precise outline 

 of the extension is not determinable, but it evidently tapers pos- 

 teriorly, and its extreme length is nearly equal to half the length of 

 the shield. 



Appendages of the character just described have already been 

 noticed by Powrie and Lankester, and, in the absence of satisfactory 

 evidence as to their connections, they have naturally been regarded 

 as pectoral fins. It now appears, however, that the structures are 

 merely a portion of the shield itself, divested of the outer and inner 

 layers to ensure flexibility. The arrangement and peculiarities of 

 the compound plates have, indeed, suggested to Prof. Lankester that 

 these appendages " may have had other functions than that of mere 

 locomotion ;" and he adds, as not improbable, " that they may have 

 been efficient in causing currents of water to pass to the branchial 

 organs covered in by the great head-shield (whose outlets are indi- 

 cated by the lateral perforation in the shield of Pteraspis), and have 

 thus aided respiration as well as locomotion, as is observed in the 

 fry of Teleostean fishes at the present day with regard to the pectoral 

 fin." Some connection with the gills has thus already been suspected, 

 and it now seems most probable that the appendages in question 

 actually correspond to a pair of opercula, and may henceforth be 

 designated as such. 



The inferior surface of the head is only shown in part by one fossil, 

 and the structures are not sufficiently complete to give any clue as 

 to the characters of the mouth. At least posteriorly, the skin is 

 supported by thin and delicate polygonal plates, closely fitted to- 

 gether (see PI. X. fig. 2). 



As in the more typical members of the genus Cep7ialaspis, the por- 

 tion of trunk behind the shield is trihedral in form, gradually tapering 

 towards the caudal extremity, and having a segmented appearance, 

 owing to the shape and arrangement of the scutes. As shown dis- 

 tinctly by one specimen (PI. X. fig. 4) and less distinctly by others, 

 the inferior surface is formed by a single paired series of elongated 

 scutes (v.), each broadest at its outer extremity and gradually nar- 

 rowing while directed forwards mesially. A series of small scutes 

 corresponding in number, and forming a kind of fringe, is arranged 

 along the inferior lateral angle (I.) of both sides. Each of these is 

 only connected with the one in front and behind at its base, and the 

 outer free extremity is directed backwards, while the anterior 

 margin is gently rounded and serrated. Again of equal number, and 

 having bevelled ends articulating with the inferior marginals just 



