PALEOZOIC TIME — CARBONIC. 725 



that, witli this admission, the thorax and head of an Insect are essentially 

 homologous with the head of a Tetradecapod Crustacean. 



Vertebrates. 



1. Fishes. — The Pteraspid section of the Placoderms, having long verte- 

 brated tails fitting them to be fleet scullers, commenced (according to the 

 present state of the facts) in the Lower Silurian (page 509). Cotempo- 

 raneously (the same locality attesting) there were normal Ganoids, the 

 Crossopterygian, which till recently were supposed to have made their first 

 appearance in the Devonian. Along with these there probably existed also 

 the Chimseroids, precursors of the Selachians, — a type of primitive features 

 now almost extinct. 



The Devonian adds to these paleozoic tribes the Brachiate Placoderms, 

 admirably armor-clad fishes. But they were short in body, and hence poor 

 at sculling, but were furnished with pectoral limbs in the shape of arms that 

 were seemingly fitted for crawling and grubbing over muddy or sandy 

 bottoms rather than for swimming. Although the appendages are called 

 "arms," and the Fishes were in appearance ''brachiate" (Fig. 982, page 

 624), the pectoral fins (to which they correspond) are homologous with the 

 hands in Vertebrates and not with the arms. They were a poor equip- 

 ment for either aquatic or terrestrial service, and the species end with the 

 Devonian. 



At the same time the Devonian waters were full, as has been shown, of 

 Selachians, Dipnoans, and typical Ganoids, of great diversity in characters, 

 and many of them unsurpassed at any later time in magnitude. 



Fishes appear to have reached their highest grade of vertebrate structure^ 

 and thus to have culminated in the Dipnoans, — species that have not only 

 lungs for breathing, as well as gills, but also, in the Ceratodus, a genus dating 

 from the Carboniferous, a finger-like jointed midrib to the pectoral fin 

 (Archypterygian), with jointed branches diverging from either side of it. 



No records of the precursors of Placoderms, Ganoids and Sharks have yet 

 been found in the rocks. The little Amphioxus, of existing seas (page 418), 

 is supposed to represent one of the early forms, because, while having the 

 general characteristics of the class, it is strikingly like an Invertebrate in 

 part of its embryological development. The Ascidians are probably degen- 

 erate forms, as held by_Lankester, derived from some species of still 

 lower grade. 



All Fishes are in several ways eminently rmdtipUcate species. This is 

 seen in the number of vertebrae ; of articulations in the limbs when articula- 

 tions exist; of teeth, and of tooth-bearing parts in the mouth. 



2. Amphibians and Reptiles. — The line from the Fishes to the Amphibians 

 is supposed to have been from the Dipnoan section. The resemblance in 

 Amphibians to the Ganoids generally is in many respects close, it extending 

 even to the form and structure of their labyrinthine teeth ; and the Dipnoans 



