16 



AGNATHA. 



A remarkable fossil from the Lower Devonian of Spitzbergen, 

 named Acanihaspis decipiens, has a dermal armour resembling 

 that of the Antiarchi in minute structure, and a ventral plastron 

 quite similar to that of the latter. The lateral appendages, 

 however, instead of being complex and moveable, are simple 

 and fixed. As shown in the accompanying diagram (fig. 16), 



FIG. 16. 



Acanthaspis decipiens ; diagram of body-shield, ventral aspect, less than half 

 nat. size. L.Devonian; Spitzbergen. ap., lateral appendage in form of 

 spine; pr., process supporting appendage. 



the hollow plate forming this appendage on each side is firmly 

 fixed to a lateral prominence of the body armour. It is, in fact, 

 merely a fixed spine. The paired appendages of the Antiarchi 

 are therefore very probably not homologous with those of the 

 Class Pisces. 



