48 FISH GALLERY. 



The earliest fossil remains referred to this class are found in the 

 Lower Silurian, in the form of small horny bodies which have been 

 regarded as teeth of Cyclostomes or Lampreys. But the first 

 undeniable evidence of a Fish, probably a Plagiostome, occurs in 

 the Upper Silurian ; from the Devonian to the Cretaceous, Ganoids 

 were extremely abundant and exhibited an endless variety of forms, 

 many of which recall, with regard to external appearance, the 

 Teleosteans of the present time ; from the former formation started 

 also Chondropterygians and other Palseichthyes ; in the Tertiary 

 Epoch the Teleosteans almost entirely replaced the Ganoids, and 

 have continued to be the predominant type of Fishes down to 

 our times. 



Fishes are distributed over all the waters of the globe, and may, 

 on the whole, be divided into Freshwater and Marine forms. 

 However, a sharp line cannot be drawn between these two kinds of 

 Fishes, for there are not only species which can gradually accom- 

 modate themselves to a sojourn in either salt or fresh water, but 

 there are also such as seem to be quite indifferent to a rapid change 

 from one to the other, as, for instance, Sticklebacks and some species 

 of Clupea, or Herrings. Further, Fishes belonging to freshwater 

 genera descend rivers and sojourn in the sea for a more or less 

 limited period ; whilst others annually or periodically ascend 

 rivers for the purpose of spawning — for instance, the Salmon and 

 many Sturgeons. Marine Fishes fall, with regard to their life and 

 distribution, into three divisions : — 1. Shore Fishes, that is, 

 Fishes which inhabit chiefly parts of the sea in the immediate 

 neighbourhood of land or banks ; 2. Pelagic Fishes, which inhabit 

 the surface and uppermost strata of the open ocean, and approach 

 the shores only accidentally or occasionally (in search of prey), or 

 periodically (for the purpose of spawning) ; 3. Deep-sea Fishes, 

 which inhabit such depths of the ocean as to be but little or not 

 at all influenced by light or the surface temperature, and which, by 

 their organization, are prevented from reaching the surface stratum 

 in a healthy condition. But it must not be imagined that these 

 three divisions are more sharply defined than Freshwater and 

 Marine Fishes, and, like these latter, they gradually pass into each 

 other. 



A number of Skeletons are exhibited in the wall-cases and table- 



