180 THE STORY OF FISH LIFE. 



have the tribasal arrangement which we referred 

 to on p. 177, fig. 7, p. 66. This tribasal form of fin 

 presents a close resemblance to the basal carti- 

 lages of the modern sharks, a fact of great 

 interest and significance. The scales of these 

 fishes are very thick and dense ; quadrangular in 

 form, therefore not overlapping, and coated with 

 " ganoine." Very little is known of the breed- 

 ing habits of these wonderful mail-clad fishes. 

 The young have very large and well-developed 

 external gills (fig. 3). 



The "ray-finned fishes," the Adinopterygii^ 

 embrace the whole of the remaining forms to 

 be discussed in this chapter. For the fishes 

 belonging to this order we may adopt Mr 

 Lydekker's name of " fan-finned," since the rays 

 or bony supports of the fin spring from a common 

 base, fan-wise, instead of being distributed so as 

 to form a fringe to a more or less extensive 

 scale-covered lobe. There are, of course, other 

 additional characters peculiar to the fan-finned 

 fishes, but these need not concern us here. 



It is a remarkable fact, but nevertheless true, 

 that the fan-finned are as old as the oldest of 

 the fringe-finned fishes, occurring as far back as 

 the Devonian period. And, furthermore, it 

 seems to be equally true that we have in the 

 modern sturgeons the highly specialised descen- 

 dants of the earliest forms of the fan-finned 

 group. For this we have the authority, so often 

 quoted here, of Dr Smith Woodward. 



These ancestral sturgeons difi'er much from 

 their modern descendants. The latter have in 

 the course of ages undergone great specialisation, 



