342 THE ORIGIN 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 trabecule, 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 
Palieostraca 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 Schimkéwitsch 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 
Ammoccetes; 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 Ammoccetes, something 
intermediate between those of the paleostracau and Ammoccetes. 
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 Asterolepide—large, oar-like 
appendages which may well represent the ectognaths. 
