RECORD OF LIFE IN THE ROCKS. 173 



255. Animal Fossils. The study of fossil zoology is 

 more satisfactory than that of fossil botany, for three 

 reasons. First, the distinctions between different kinds 

 of animals are more readily made out from the partial re- 

 mains which the rocks present to us. The reason is that 

 the differences between animals are of a more decided 

 character, and especially those which mark the grand di- 

 visions into classes, as you will soon see. Secondly, most 

 animal remains are in a better state of preservation than 

 vegetable remains. Shells, corals, scales, teeth, and bones 

 are very durable. Thirdly, more attention has been paid 

 to fossil zoology than to fossil botany by geological ob- 

 servers. 



256. Protozoans. This term, derived from two Greek 

 words, protos, first, and zoon, animal, is applied to a 

 class of organisms which seem to stand on doubtful 

 ground, being half vegetable and half animal in their 

 characteristics. They are the sponges, the foraminifera, 

 and the infusoria. They have, for the most part, an in- 

 distinct organization, so that they can not be classified 

 with any of the great divisions of the animal kingdom. 

 Professor Dana therefore speaks of them as systemless ; 

 that is, as belonging to no system. And yet it would 

 seem as if some of them have a sufficiently definite form 

 to be classified, as operculina, /, polymorphina, g, and 

 nummulites, >, in Fig. 83, which are certainly very much 

 like ordinary shell-fish, though their resemblance in cer- 

 tain respects to polypes may render it improper to lo- 

 cate them with shell-fish in a definite classification. The 

 sponges have been thought by some to be on doubtful 

 ground, but it is now considered settled that they are 

 animal. The flinty diatoms, on the other hand, as stated 

 in 240, are now to be placed among vegetable organ- 

 isms, and are not, therefore, any longer to be called ani- 

 malcula?. 



I pass now to the consideration of those animals that 

 can be definitely classed in systems. 



