342 THE O RIG IX OF VERTEBRATES 



I venture, then, to suggest that in the Osteostraci the median 

 hard plate or glabellum protected a brain which was enclosed in a 

 membranous cranium, very probably not yet complete in the dorsal 

 region — certainly not complete if the median pineal eyes so univer- 

 sally found in these ancient fishes were functional — a cranium derived 

 from the basal trabeculse, in precisely the same manner as we see it 

 already in its commencement in Phrynus and other scorpions. With 

 the completion of this cranium and its conversion into cartilage, and 

 subsequently into bone, an efficient protection was afforded to the 

 most vital part of the animal, and thus the hard head-shield of the 

 Palaeostraca and of the earliest fishes was gradually supplanted by 

 the protecting bony cranium of the higher vertebrates. 



Step by step it is easy to follow in the mind's eye the evolution 

 of the vertebrate cranium, and because it was evolved direct from 

 the plastron, the impossibility of resolving it into segments is at 

 once manifest ; for although the plastron was probably originally 

 segmented, as Schimkewitsch thinks, all sign of such segmentation 

 had in all probability ceased, before ever the vertebrates first made 

 their appearance on the earth. 



It follows further, from the comparison here made, that those 

 antero-lateral markings indicative of segments, found so frequently 

 in these primitive fishes, must be interpreted as due not to gills but 

 to aponeuroses, due to the presence of muscles which moved proso- 

 matic appendages, muscles which arose from the dorsal region in 

 very much the same position as do the muscles of the lower lip in 

 Ammocoetes ; the latter, as already argued, represent the tergo-coxal 

 muscles of the last pair of prosomatic appendages — the chilaria or 

 metastoma. Such an interpretation of these markings signifies that 

 the first-formed fishes must have possessed prosomatic appendages of 

 a more definite character than the tentacles of Ammocoetes, something 

 intermediate between those of the paiasostracau and Ammocoetes. 



For my part I should not be in the least surprised were I to hear 

 that something of the nature of appendages in this region had been 

 found, especially in view of the well-known existence of the pair of 

 appendages in the members of the Asterolepidre — large, oar-like 

 appendages which may well represent the ectognaths. 



