DEVONIAN PERIOD 39 



bnsthng with tentacles, were also swimming about. These cyclosto- 

 creatures were not true fishes ; for they had no paired fins ; mata 



nor were their mouths surrounded with any hard material 

 constituting jaws. The largest of them, so far as is known, 

 did not exceed two inches in length : and their bodies seem 

 to have been unprotected by any plates or scales (Palceo- 

 spondylus). In anatomical respects they resembled lampreys ; 

 but they were more highly developed than those animals! 

 Possibly they represented that line of life before it entered on 

 a path of degeneracy. 



Other and much larger forms were partially armoured, ARTHRO- 

 and were provided with toothed jaws, and also, at least on dirans 



the hinder part of the body, with paired fins (Coccosteus). 

 These true fishes were peculiar in being able to raise and 

 depress the head without moving the body — hence their 

 name the " joint-necked " (Arthrodirans). The nature of 

 their teeth suggests a relationship with lung-fishes. They 

 had, however, become too far specialised to be of any service 

 in the work of Evolution. They represented, in short, an 

 unsuccessful divergence, as did the armoured semi-fishes ; 

 and they experienced a like fate at the close of the Period, 

 or soon after. In North America some of the later forms 

 measured fully twenty feet in length (Dinicthys Herzeri). 

 These nodding giants were quite without teeth, and the jaws 

 had been transformed into a sort of nipper and nutcracker 

 apparatus. 



These various vertebrates, therefore, were, so to speak, 

 in the shade. Indeed, the lamprey-like organism was the only 

 bright spot among them. But fish-life, though destined to 

 come to naught in some of its developments, had made great 

 advances in other directions. This state of affairs is brought 

 out in bold relief in the geological record : but there is a 

 remarkable lack of evidence as to the intervening stages of 

 the progress. 



Shark-like fishes, in some evidence in Silurian times, were sharks 

 now represented by several species. The biggest forms 

 varied in length from two to five feet (Cladoselache). As 

 regards skull and jaws they resembled the cow-sharks (Noti- 

 danidcB) of our own time, but in other respects they were 



