03° 



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 



Fig. 135. — Diagrams to show the different shapes of Head-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- 

 ccetes 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 



