GEOLOGICAL APPEARANCE. lxxxi 



which light penetrates more or less as in the littoral zone, where sea- weeds, 

 corals and shells are found and which may be termed the fauna of light : 

 whereas beyond this zone a region of darkness obtains. Pelagic species are 

 normally observed to be more or less restricted to certain zones of depth or 

 bathymetrical regions and are known as surface, median or abyssal forms. 

 While fishes of the fresh-waters are found frequenting rivers or fluviatile 

 forms, or else passing their lives in lakes or ponds or lacustrine species : 

 such as reside in large pieces of water may likewise select the shores and 

 surface waters or the deeper portions of the lake. 



GEOLOGICAL APPEARANCE. 



Among the many interesting questions relating to fish, assuredly not the 

 least so is when did they first appear in the waters of the globe which we 

 inhabit ? And for an answer we have to ask the Geologists, who assert 

 that the lowest type of vertebrate life first occurred near the close of the 

 Silurian epochs at the base of the Ludlow rocks, where certain spines, jaws, 

 teeth and a cephalic buckler of probably a Pteraspis, as well as coprolite bodies 

 containing crinoids and molluscs, have been obtained, but no other portions 

 of true piscine skeleton. In the " Devonian " or " Old Eed Sandstone," 

 remains of fish in a more perfect state and in larger numbers are found, so 

 much so that this strata has been termed the "age of fish." Some of the 

 Silurian forms seem to have continued into the Devonian, and at this period 

 ganoids and sharks became predominant but no trace of segmentation of the 

 vertebra has been observed ; ganoids and sharks are very abundant in the 

 Carboniferous and Permian rocks which show the end of the Pakeozoic epoch. 



Up to the Mesozoic epoch fishes possessing heterocercal tails (page ix) 

 were the ordinary type, but now they commenced to diminish, and the 

 homocercal (page ix) form began to obtain predominance ; in short, in the 

 earliest times the posterior termination of the vertebral column was very 

 similar to what we now find as an embryonic condition of teleosteans, 

 persistent formerly, transitory now. The fishes in the Lias besides developing 

 homocercal tails commence to show ossification and segmentation of the 

 vertebra3, and Leptolepis a teleostean form, has been supposed to have 

 been discovered. Among the fishes of the Oolitic strata a further advance 

 towards the fauna of this period occurs, while rays become more numerous. 

 In the chalk or cretaceous era, although Chondropterygians are found, it is the 

 Teleosteans or bony forms, as already observed, that obtain predominance, 

 and so on through the Tertiary period to our times. 



It has been pointed out that had such fish as Cyclostomata or Lepto- 

 cardii existed in Silurian or Mesozoic epochs it would hardly be expected that 



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