94 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



Starfish, bivalve Molluscs with thin and gaping 

 shells, Pteropods with very fragile shells, Gastro- 

 pods, like the Bellerophon and the Pleurotomary, 

 whose shell carries a slit the whole length of the 

 spire, and Cephalopods like the Nautilus, with widely 

 opened chamber, marine Worms which dig, as at 

 the present day, holes in the sand, and those great 

 Crustaceans, the Merostomes, with a rather thin and 

 chitinous carapace. The Carboniferous era offers us 

 the Arachnides and the Insects. The cartilaginous 

 and naked fishes, akin to our Dog-fish, abound in all 

 Primary deposits, and have only been preserved to 

 us, thanks to their teeth and the spines of their fins. 

 There are reasons for thinking that entirely soft, 

 low-class Vertebrates, analogous to our Lampreys, ex- 

 isted in the Cambrian Sea. Lastly, a few puny 

 Amphibians of the Permo-carboniferous epoch were 

 as little protected as are our Batrachians. In 

 reality, naked, or but slightly protected, animals 

 are not rare in early geological deposits, and if they 

 are not still more common, it is for the very simple 

 reason that their preservation is difficult, and 

 that most of them have disappeared without leaving 

 any traces. We might even go further, and by a 

 converse reasoning to that of Gaudry, show that 

 the best fortified animals, the Cystidea, the tesselated 

 Crinoids, and the armoured Fishes have had a less 

 powerful vitality than that of the other groups, and 

 have died out more rapidly, giving place to groups 

 of animals apparently less well protected against 

 the attacks of their enemies. 



Other forms of progress recognized by Gaudry 

 are no more in conformity with the evidence of 



