338 THE ORIGIN OF VERTEBRATES 
muscles, the shape of the muco-cartilaginous head-shield will vary, 
and if it were calcified and then fossilized we should obtain fossil 
head-shields of widely differing configuration, although such fossils 
might be closely allied to each other. This is just what is found 
in this group. Let the muco-cartilage extend over the whole of 
the branchial region of Ammoccetes, the resulting head-shield would 
be as in Fig. 135, A; the branchial bars below the muco-carti- 
laginous shield might or might not be evident, and the line between 
the branchial and the trigeminal region might or might not be 
indicated. Such a head-shield would closely resemble those of Didy- 
maspis and Tremataspis respectively. Now suppose the somatic 
musculature to encroach slightly on the branchial region and also 
) 
i nie i 
‘a fa 
a | 
a | 
i Bact 
P nu 
i a t 
sf Li a tt 
} l 
i Ct 
Fic. 135.—D1aGRAMS TO SHOW THE DIFFERENT SHAPES OF HEaApD-SHIELDS DUE TO 
THE FORWARD GROWTH OF THE Somatic MuscuLaTuRE. 
A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammoccetes. 
laterally to the end of the anterior branchial region, then we should 
obtain a shape resembling that of Thyestes (Fig. 135, B). Continue 
the same process further, the lateral muscle always encroaching 
further than the median masses, until the whole or nearly the whole 
branchial region is invested, and we get the head-shield of Cephalaspis 
(Fig. 135, C); further still, that of Keraspis, and yet still further, 
that of Ammoccetes (Fig. 135, D). 
So close is this similarity, from the comparative point of view, 
between the dorsal head-shield of the Osteostraci and the dorsal 
cephalic region of Ammoccetes that it justifies us in taking Ammo- 
coetes as the nearest living representative of such types; it is justifi- 
able, therefore, to interpret by means of Ammoccetes the position of 
other organs in these forms. First and foremost is the hard plate 
