i86 EVOLUTION OF ELASMOBRANCHS vi. 8- 



meeting the circumstances certain types will be suitable at one time, 

 others at another. We know that genetic variations will produce fluctua- 

 tions of type — at a time when circumstances force the animals to strive 

 in one direction those with a particular bodily type, say, broad, 'hetero- 

 dont' teeth will be selected. When fish food again becomes available 

 those animals born with quicker habits and sharper teeth will be able 

 to eat the fish and the shark type returns. 



The method of ensuring stability in the pitching plane adopted by 

 elasmobranchs (p. 136) necessitates a certain flattening of the front 

 end of the animal. It is not therefore surprising that this tendency is 

 often exaggerated and has several times produced flattened bottom- 

 living creatures, such as the skates and rays. The Actinopterygii 

 show the opposite tendency, to lateral flattening (p. 248). We might 

 imagine that most of the modern skates and rays had become so 

 modified in structure that only life on the bottom is possible for them 

 and that there could be no return to a free-swimming, fish-eating 

 habit, but it would not be true to say that this is certain or that the 

 past history of the group shows undoubted evidence of such irrever- 

 sible specialization. 



The only general conclusion from our study of elasmobranchs since 

 the Devonian, then, is that they have tended to keep alive by eating 

 fish or invertebrates, that some have changed little during this time 

 but that, judging especially from the modern forms, the group tends 

 to produce varied types at any one time, each able to find its food in a 

 special manner. It is not clear that the group has advanced, in any 

 sense, since the Devonian. The type has always been a successful one, 

 able to produce specialized carnivores. We do not know enough to be 

 sure whether the number of creatures with this organization has 

 changed greatly, but it seems that, except for a reduction in numbers 

 in the Triassic, they have always been moderately abundant and are 

 perhaps at present on the increase. 



9. The earliest Gnathostomes, Placoderms 



*Class Placodermi (= Aphetohyoidea) 



*Order 1. Acanthodii. Silurian-Permian (*Climatius) 

 *Order 2. Arthrodira. Silurian-Devonian (*Coccosteus) 

 *Order 3. Macropetalichthyida. Devonian (*Lunaspis) 

 *Order 4. Antiarchi (= Pterichthyomorphi). Devonian (*Bothri- 



olepis) 

 *Order 5. Stegoselachii. Devonian-Carboniferous (*Gemundina) 

 *Order 6. Palaeospondyli. Devonian {* Palaeospondylus) 



