i 3 o VERTEBRATES WITHOUT JAWS iv. 20 



extensions on each side of the head but no paired fins. The only 

 median fin was the anal. 



The affinities of these ostracoderm fossils with each other and with 

 the cyclostomes have been much disputed. Lankester claimed that 

 pteraspids were related to cephalaspids 'because they are found in the 

 same beds, because they have a large head shield and because there is 

 nothing else with which to associate them'. At the other extreme 

 Stensio holds that we have sufficient evidence to assert that the 

 pteraspids have given rise to the myxinoids, and the cephalaspids to 

 the lampreys. Except for the absence of jaws there is indeed little in 

 common among the fossil forms. The differences in the shape of the 

 tail are especially baffling. As White points out, an animal with a 

 heterocercal tail and pectoral fins can hardly have lost either of these 

 organs independently. He suggests that the earliest vertebrates pos- 

 sessed straight ('diphycercal') tails and that from these were evolved 

 on the one hand the pteraspids with hypocercal tails and on the other 

 the cephalaspids with upturned heterocercal tails. The modern cyclo- 

 stomes are perhaps derived from the latter, but which, if either, group 

 gave rise to the earliest gnathostomes is unknown. 



The Agnatha were the first animals of the chordate type to become 

 large, and they apparently all did so by feeding on the detritus at the 

 bottom of rivers and lakes. They evolved into various types, mostly 

 rather heavily armoured and perhaps slow-moving forms. The lam- 

 preys and hag-fishes have been derived from early Agnatha by the 

 evolution of a sucking mouth, perhaps with loss of the bony skeleton 

 and paired limbs. However, it was the unknown forms that evolved 

 a biting mouth that made the next great advance in vertebrate 

 evolution. 



