344 TIIE ORIGIN OF VERTEBRATES 



covering of the Osteostraci, with the consequent supposition that 

 their ancestors possessed a cartilaginous covering This argument is 

 entirely upset, if, as I have pointed out, the structure of the cepha- 

 laspid shield is that of muco-cartilage and not of hone. If these 

 plates are a calcified muco-cartilage. then the whole argument for 

 their ancestry from animals with a cartilaginous skeleton falls to the 

 ground, for muco-cartilage is the precursor not only of hone, but also 

 of cartilage itself. 



The evidence, then, points strongly in favour of Cope's view that 

 the most primitive fishes were Agnatha, after the fashion of cyclo- 

 stomes, as is also helieved by Smith Woodward, Bashford Dean, and 

 Jaekel. 



Among living animals, as I have shown, the Limulus is the sole 

 survivor of the palseostracan type, and Ammoco'tes alone gives a 

 clue to the nature of the cephalaspid, i.e. the osteostracan fish. Older 

 than the latter is the heterostracan, Pteraspis, and Cyathaspis. Is 

 it possible from their structure to obtain any clue as to the actual 

 passage from the pala'ostracan to the vertebrate ? 



Here again, as in the case of the Osteostraci, a relationship to the 

 elasmobranch has been supposed, for the following reasons : — 



The latest discoveries in the Silurian and Devonian deposits have 

 brought to light strange forms such as Thelodus and Drepanaspis, of 

 which the latter from the Devonian must, according to Traquair, be 

 included in the Heterostraci. It possessed, as seen in Fig. 139, large 

 plates, after the fashion of Pteraspis, and also many smaller ones. 



The former, from the upper Silurian, belongs to the CYelolepida?, 

 and was covered over with shagreen composed of small scutes, after 

 the fashion of an elasmobranch. Traquair suggests that Thelodus 

 arose from the original elasmobranch stock; that by the fusion of 

 scutes such a form as Drepanaspis occurred, and, with still further 

 fusion, Pteraspis. 



There are always two ways of looking at a question, and it seems 

 to me possible and more prol table to turn the matter round and to 

 argue that the original condition of the surface-covering was that of 

 large plates, as in Pteraspis. By the subsequent splitting up of such 

 plates, Drepanaspis was formed, and later on, by further splitting, 

 the elasmobranch, Thelodus beincr a stage on the way to the forma- 

 tion of an elasmobranch, and not a backward stage from the elasmo- 

 branch towards Pteraspis. 



