96 VESTIGES OF THE 



in the rock. Singly they are the most unimportant of 

 all animals, but in the mass, forming as they do such 

 enormous strata over a large part of the earth's surface, 

 they have an importance greatly exceeding that of the 

 largest and noblest of the beasts of the field. Moreover, 

 these species have a peculiar interest, as the only specific 

 types of that early age which are reproduced in the 

 present day. Species of sea mollusks, of reptiles, and of 

 mammifers, have been changed again and again, since the 

 cretaceous era ; and it is not till a long subsequent age 

 that we find the first traces of any other of even the 

 humblest species which now exist ; but here have these 

 humble infusoria and polythalamia kept their place on 

 earth through all its revolutions since that time — are 

 we to say, safe in their very humility, which might adapt 

 them to a greater variety of circumstances than most 

 other animals, or are we required to look for some other 

 explanation of the phenomenon ? 



All the ordinary and more observable orders of the 

 inhabitants of the sea, except the cetacea, have been 

 found in the cretaceous formation — zoophytes, radiaria, 

 mollusks, Crustacea (in great variety of species), and 

 fishes in smaller variety. Down to this period, the 

 placoid and ganoid fishes had, as far as we have evidence, 

 flourished alone ; now they decline, and we begin to find 

 in their place fishes of two orders of superior organisa- 

 tion, the orders which predominate in the present creation. 

 These are osseous in internal structure, with corneous 

 scales, the latter being circular in the one case, and 

 pectinated or indented at one side in the other ; hence 

 the two orders are called respectively cycloid and ctenoid 

 by M. Agassiz, who, as has been remarked, asserts that 

 the outer covering of fishes is a suflicient indication of 



