S62 POPULAR SCIENCE MONTHLY. 



from which diverge radius and ulna, tlie carpal bones being formed 

 of the intervening cartilage. 



From the crossopterygians springs the main branch of true fishes, 

 known collectively as Actinopteri, those with ordinary rays on the paired 

 fins instead of the jointed archipterygium. The transitional series of 

 primitive Actinopteri is here called by the name of Ganoid. The 

 ganoid differs from the Crossopterygian in having basal elements of the 

 paired fins small and concealed within the flesh. But other associated 

 characters of the Crossopterygii and Dipnoi are preserved in most of 

 the species. Among these are the mailed head and body, the hetero- 

 cercal tail, the cellular air-bladder, the presence of valves in the arterial 

 bulb, the presence of a spiral valve in the intestine and of a chiasma 

 in the optic nerves. All these characters are found in the earlier types 

 so far as known, and all are more or less completely lost or altered 

 in the teleosts or bony fishes. Among the existing ganoids the gar-pike 

 (Lepisosteus) is the last of a long series of Mesozoic forms of the same 

 general structure. The gar-pikes are cylindrical or arrow-shaped. 

 Among these early types is every variety of form, some of them being 

 almost as long as deep, and every intermediate form being represented. 

 An offshoot from this line is the bow-fin (Amia calva), perhaps the 

 closest living ally of the bony fishes, showing distinct affinities with the 

 great group to which the herring and salmon belong. Near relatives of 

 the bow-fin fiourished in the Mesozoic, among them some with a forked 

 tail, and some with a very long one. From forms of this type the body 

 of recent fishes may be descended. 



Another branch of ganoids, widely divergent from both gar-fish and 

 bow-fin and not recently from the same primitive stock, included the 

 sturgeons {Acipenser, Scapliirliynchus, Kessleria) and the paddle-fishes 

 (Polyodon and Psephurus). These differ widely from any other types, 

 recent or fossil, showing features of degeneration as compared with 

 their extinct ancestors, while again sturgeon and paddlefish differ widely 

 from, each other. It has been suggested that the cat-fishes (Siluridae) 

 are descended from the sturgeons, but the resemblance vanishes the 

 more closely the groups are compared, nor are we anywhere sure of the 

 point where any part of the teleost series is Joined to the ganoids. We 

 can only say that the sturgeons are more or less degraded ganoids with 

 cartilaginous skeletons, of unknown derivation and of unsettled rela- 

 tionships. 



All other fishes have ossified instead of cartilaginous skeletons. The 

 dipnoan and ganoid traits one by one are more or less completely lost. 

 Through these the main line of fish development continues and the 

 various groups are known collectively as bony fishes or teleosts. 



The earliest of the true bony fishes or teleosts appear in Mesozoic 

 times, the most primitive forms being soft-rayed fishes with unmodi- 



