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 Mollusca; 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. Ther* 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 



