IX GEOLOGICAL CONTEMPORANEITY 291 



animals so distinct from those now living as to 

 require orders for themselves; and these do not 

 amount, on the most liberal estimate, to more than 

 about 10 per cent, of the whole. 



There is no certainly known extinct order of 

 Protozoa ; there is but one among the Coelenterata 

 — that of the rugose corals ; there is none among 

 the MoUusca; there are three, the Cystidea, 

 Blastoidea, and Edrioasterida, among the Echino- 

 derms; and two, the Trilobita and Eurypterida, 

 among the Crustacea ; making altogether five for 

 the great sub-kingdom of Annulosa. Among 

 Vertebrates there is no ordinally distinct fossil 

 fish : there is only one extinct order of Amphibia 

 — the Labyrinthodonts ; but there are at least four 

 distinct orders of Reptilia, viz. the Ichthyosauria, 

 Plesiosauria, Pterosauria, Dinosauria, and perhaps 

 another or two. There is no known extinct order 

 of Birds, and no certainly known extinct order of 

 Mammals, the ordinal distinctness of the " Toxo- 

 dontia " being doubtful. 



The objection that broad statements of this 

 kind, after all, rest largely on negative evidence is 

 obvious, but it has less force than may at first be 

 supposed ; for, as might be expected from the cir- 

 cumstances of the case, we possess more abundant 

 positive evidence regarding Fishes and marine 

 Mollusks than respecting ' any other forms of 

 animal life ; and yet these offer us, through the 

 whole range of geological time, no species ordinally 



