426 THE CARBONIFEROUS PERIOD 



such as Cycloceras, Trigonoceras, etc. The Nautiloids have shells 

 ornamented with prominent ridges or tubercles. The Ammonoids 

 continue to be represented by Goniatites (V, n), but the Carbo- 

 niferous species of this genus display an advance over those of the 

 Devonian in the greater complexity of their sutures, looking for- 

 ward to the remarkable condition attained in Mesozoic times. 



Vertebrata. — It is in this group that the most marked advances 

 of Carboniferous life are to be observed, and the incipient stages 

 of Mesozoic development are clearly shown. The extraordinary 

 and bizarre Ostracoderms have become extinct, though the Arthro- 

 dirans continue into the coal measures. 



The Selachians are numerous and varied, and some of them 

 highly specialized. Acanthodes is a small shark covered with a 

 dense armour of exceedingly minute square scales, and the fins are 

 supported by a heavy spine along their anterior borders. Another 

 remarkable shark is Pleuracanthus (a Permian species is shown 

 in Fig. 145), which has many features in common with the Dipnoi, 

 such as the shape of the tail, the character of the pectoral fins, and 

 the bones which form a roof for the skull, while the skin is naked. 

 Isolated fin-spines and teeth show that many other kinds of sharks 

 existed in the Carboniferous, in some of which the teeth were con- 

 verted into a crushing pavement, adapted for a diet of shell-fish. 



The Dipnoi continue, though in diminished numbers, and their 

 most prominent representative is the genus Ctenodus. 



The Crossopterygia?is are much less abundant than in the De- 

 vonian ; the commonest American genus is Ccelacanthus, which, 

 though unmistakably a member of this group, has assumed the 

 form of a bony fish, and looks much like a chub. 



The Actinopterygians are still represented only by the Ganoid 

 suborder ; these hold their own and even increase their numbers, 

 many new genera replacing those of Devonian times. Earylepis, 

 Palceoniscus, Eurynotits, and Cheirodus are the best known gen- 

 era ; they are all of moderate size and in appearance are not strik- 

 ingly different from modern fishes. 



The Amphibians, which we have seen reason to believe existed 

 in the Devonian, are of greatly increased importance in the Car- 



