IV. 20 



FOSSIL AGNATHANS 



125 



evolution, probably that of about the Silurian period. Their special 

 interest for us is in giving an insight into the organization possessed 

 by the vertebrates before jaws were evolved. However, no doubt many 

 changes have gone on during cyclostome evolution and we must not 

 suppose that all Silurian vertebrates were like lampreys. Indeed, we 

 may now complete our picture of this stage of evolution by examining 

 the fossil fishes known to have existed at that period. We shall find 

 them superficially so different from modern cyclostomes that only 

 careful morphological comparison reveals the similarities. The inquiry 

 will show us once again how a common plan of organization can be 

 found in animals of very different superficial form and habits. 



Fig. 83. A ccphalaspid restored (Hemicyclaspis). 



d. dorsal fin; bf. lateral field; pec. pectoral fin; p. pineal; sclr. sclerotic ring. 

 (From Stcnsio.) 



20. Fossil Agnatha, the earliest-known vertebrates 



The ostracoderms are fossil forms from freshwater Silurian and 

 Devonian deposits. They are therefore the oldest fossil vertebrates 

 known to us (except for a few Ordovician fragments), and this makes 

 it specially interesting that they show affinity with the cyclostomes. 

 These are fossils that are rarely found complete, particularly the 

 pteraspids, but a quarry in Herefordshire yielded numerous whole 

 specimens of Cephalaspis and Pteraspis of Old Red Sandstone age, 

 probably all from a single dried-up pool. 



In the cephalaspids (Osteostraci) the head was flattened and com- 

 posed largely of a shield. The rest of the body was fish-like, with an 

 upturned tail (heterocercal, see p. 136) covered with heavy bony scales 

 (Fig. 83). A pair of flaps behind the gills may have functioned like 

 pectoral fins. 



On the dorsal surface of the shield are two median holes, one 

 behind the other, which served a naso-hypophysial opening and a 

 pineal eye. The whole outline of the cranial cavity is preserved and 

 shows a brain remarkably like that of a lamprey, with a naso-hypo- 

 physial canal below it (Fig. 84). There were paired eyes and only two 



