328 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. 
B 
Fic. 127.—A, Dorsan Heap-SHIELD OF CEPHALASPIS (from LankESTER); B, Dorsau 
Heap-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 Rohon at Rootzikull, and 
is thus described by him :— 
The frontal part of the dorsal 
head-plate carried (Fig. 128) the 
two orbits for the lateral eyes 
(Z.¢.),.a marked frontal organ 
(fro.), and a median depression 
(gl.), to which he gives the 
name parietal organ. The oc- 
Fic. 198—Dorsat Huap-Sureip or Thy- Cipital part (oce.) was clearly 
estes (Auchenaspis) verrucosus. (From sgeomented, and carried, he 
meee) er ae ee thinks, the branchie. I repro- 
Hr nail opening: Taba vs: ly Guoe Tohon's figure of the 
cipital region. 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. 
