RELATIONSHIP OF AMMOCGTES TO OSTRACODERMS 341 
where Schmidt and Rohon located it in Thyestes, viz. the so-called 
occipital region. 
This discovery of Rohon’s is, in my opinion, of immense importance, 
for it indicates that, in these early fishes, the prosomatic segmenta- 
tion, associated with the trigeminal nerve, was much more well- 
marked than in any fishes living in the present day. Why should 
it be more well-marked? Turning to the palostracan, it.is very 
suggestive to compare the markings on their prosomatic carapace 
with these markings. Again and again we find indications of seg- 
mentation in these fossils similar to those seen in the ancient fishes. 
Thus in Fig. 138 I have put side by side the paleostracan Bunodes 
and the fish Thyestes, both life 
size. In the latter I have indicated 
Rohon’s segments; in the former the 
markings usually seen. 
From the evidence of Phrynus, 
Mygale, etc., as already pointed out, 
such markings in the palostracan 
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fossils would indicate the position of & 
the tergo-coxal muscles of the pro- A 
somatic appendages, even though fy, 138.--A, Ourzine oF Thyestes 
such appendages have not yet been Verrucosus with Rouon’s Suc- 
= a Act Sa sion Geant that MENTS INDICATED ; B, OUTLINE OF 
: ES eOVened, ane ay 28 ah pee : Bunodes Lunula with Lareran 
in all these cases there is a distinct EYEs INSERTED. 
indication of a median plate or Both figures natural size. 
glabellum in addition to the seg- 
mental markings. Especially instructive is the evidence of Phrynus, 
as is seen by a comparison of Figs. 107 and 108, which shows clearly 
that this median plate (glab.) covered the brain-region, a brain-region 
which is isolated and protected from the tergo-coxal muscles by the 
growth dorsalwards of the flanges of the plastron. In this way an 
incipient cranium of a membranous character is formed, which helps 
to give attachment to these tergo-coxal muscles. As such cranium 
is derived directly from the plastron, it is natural that it should 
ultimately become cartilaginous, just as occurs when Ammoccetes 
becomes Petromyzon and the cartilaginous cranium of the latter 
arises from the membranous cranium of the former. In Galeodes 
also the growth dorsalwards of the lateral flanges of the plastron to 
form an incipient cranium in which the brain lies is very apparent. 
