298 LOWER VERTEBRATES. 



became obliterated by the extinction of intervening fomis ; orders have become recog- 

 nizable and definable, and are expressive of our knowledge as well as of our ignorance, 

 for trenchantly definable groups are a measure of our ignorance of intermediate 

 terms. 



Compare, for a moment, the faunas of the present and the past. 



The predominant forms of our times are Acanthopterygians, Malacopterygians, 

 Plectospondyles, Nematognaths, and Apodals, and offshoots from them. A single 

 species (Amia calva) represents one old order, — the Cycloganoids ; a single genus 

 (Xepidosteus) remains of another once widely distributed order, — the Rhomboga- 

 noids ; a couple of genera (I'olypteriis and Calamoichthys) are left of a still more 

 archaic order — the Crossopterygians, and three scattered genera survive of another 

 formerly rich old-fashioned type, — the Dipnoans. Such are the fishes. The history 

 of the Selachians is parallel. New-fashioned types are many, old-fashioned ones few. 



Fish remains have been found near the base of the upper Silurian, but the 

 earliest fauna of which we have any approach to an adequate conception is of the 

 Devonian epoch. No Acanthopterygians, Malacopterygians, Plectospondyles, Nema- 

 tognaths, Apodals, or any of their kindred had then come into being. The predomi- 

 nant forms were most like the Dipnoans and Crossopterygians, but not very near 

 them. Among the nearest to the Dipnoans were, for example, Ctenodus and Dipterus, 

 but no existing order, properly defined, seems to have been ushered in as such so 

 early. Several curious types, such as the Acanthodines, Placoderms, Osteostracans, 

 and Heterostracans, were conspicuous, and soon to become extinct; other types 

 remain yet to be discriminated among the fishes of that period ; the Selachians were 

 represented by members of extinct archaic orders, and no sharks of the common 

 modern type, much less any rays, had become developed. No remains of Myzonts or 

 Leptocardians have been discovered, but doubtless those classes were present under 

 diversified forms. The whole fauna was a strange one, and contrasted, as well by 

 what it had as by what it had not, with the existing one. 



How and when such forms as were characteristic of the paleozoic period died out 

 and others gradually took their place cannot be told here. Suffice it to say that no 

 undoubted Teleost fishes have been found in older formations than the cretaceous, 

 although it may be that even a few triassic forms may prove to be the forerunners 

 of that great group. 



Another tendency was exhibited by others of the old forms. In the Devonian age 

 were fishes that doubtless came out on the laud, and the potentiality of the quadruped 

 limbs at least became manifest. 



In a synthesis of the still existing Polypterus and Neoceratodus we have the 

 beginning of the quadrupedal vertebrate. The undivided cartilaginous coracoid of 

 Polypterus has a tubercle articulating with diverging rods, and in the one we have 

 the rudiment of a humerus, in the other the representatives of the ulna and radius, 

 while the indifferentiated cartilage between the diverging rods is material for the 

 carpal bones, and in bones radiating from that cartilage are the homologues of the 

 metacarpals. The attempts of a j^rimitive animal of such a type to travel on land 

 might develop the fore limb, and the hind one would follow in sympathy with the 

 other. Then we would have the first of the quadruped vertebrates — the Amphibians. 



Theodobe Gill. 



