RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 34 1 



where Schmidt and Rohon located it in Thyestes, viz. the so-called 

 occipital region. 



This discovery of Eohon'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 clay. Why should 

 it be more well-marked ? Turning to the pakeostracan, 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 pakeostracan Bunodes 

 and the fish Thyestes, both life 

 size. In the latter I have indicated 

 Iiohon's segments; in the former the 

 markings usually seen. 



From the evidence of Phrynus, 

 Mygale, etc., as already pointed out, 

 such markings in the paheostracan 

 fossils would indicate the position of 

 the tergo-coxal muscles of the pro- 

 somatic appendages, even though 

 such appendages have not yet been 

 discovered, and it is significant that 

 in all these cases there is a distinct 

 indication of a median plate or 

 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 dorsal wards 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 Ammocotes 

 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. 



Fig. 138.— A, Outline op Thyestes 

 Verrucosus with Rohon's Seg- 

 ments indicated ; B, Outline op 

 Bunodes Lunula with Lateral 

 Eyes inserted. 



Both figures natural size. 



