28 



THE ORIGIN OF VERTEBRATES 



and occipital region is capable of preservation, the line of demarca- 

 tion between these two regions being well marked in the latter species. 



V p-°- v 



Fig. 127. — A, Dorsal Head-Shield of Cephalaspis (from Lankester) ; B, Dorsal 

 Head-Shield of Keraspis (from Lankester). 



In the best preserved specimens of all this group of fishes a frontal 

 median orifice is always present ; it appears in some specimens 



obscurely partially divided into 

 two parts. Perhaps the best 

 specimen of all was obtained 

 by Eohon at Rootzikiill, and 

 is thus described by him : — 



The frontal part of the dorsal 

 head-plate carried (Fig. 128) the 

 two orbits for the lateral eyes 

 (I.e.), a marked frontal organ 

 (fro.), and a median depression 

 (gl.), to which he gives the 

 name parietal organ. The oc- 

 cipital part (oce.) was clearly 

 segmented, and carried, he 

 thinks, the branchiae I repro- 

 duce Eohon's figure of the 

 frontal organ in Thyestes (Fig. 

 129) ; he describes it as a 

 deeply sunk pit, divided in the middle by a slit, which leads deeper 

 in, he supposes, towards the central nervous system. 



Fig. 128. — Dorsal Head-Shield of Thy- 

 estes (Auchenaspis) verrucosus. (From 



ROHON.) 



Fro., narial opening ; I.e., lateral eyes ; gl., 

 glabellum or plate over brain; Occ, oc- 

 cipital region. 



