344 THE 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 bone. 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 bone, 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 believed by Smith Woodward, Bashford Dean, and 
Jaekel. 
Among living animals, as I have shown, the Limulus is the sole 
survivor of the paleostracan type, and Ammoca:tes alone gives a 
clue to the nature of the cephalaspid, 7.c. 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 paliostracan 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, he 
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 Ccelolepide, 
and was covered over with shagreen composed of small scutes, after 
the fashion of an elasmobrancbh. 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 probable 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 being a stage on the way to the forma- 
tion of an elasmobranch, and not a backward stage from the elasmo- 
branch towards Pteraspis. 
