Animals Before Mem 



life of other days. It is not only the larger 

 animals that left this record in the rocks, but 

 scores of smaller, more insignificant creatures 

 — crabs, shell-fish, and even insects. Many of 

 these marks have been read by comparing them 

 with the impressions left by existing animals as 

 they crawled over mud and sand, or over wet 

 plaster of Paris; but now and then we come 

 upon markings quite different from those made 

 by any animal wdth which we are acquainted. 

 In such cases, knowing the kind of tracks made 

 by living animals and the manner in which they 

 move their legs, it is necessary to cast about for 

 some fossil form whose feet can be made to fit 

 the impressions, and in this way , were inter- 

 preted the markings on Cambrian rocks now 

 ascribed to trilobites. 



If the tracks of trilobites were puzzling 

 because they were different from those made 

 by any modern animals, those made by the 

 great reptiles called dinosaurs were long misin- 

 terpreted for precisely the opposite reasoD. 

 Ordinarily the feet of different groups of ani- 

 mals are constructed on different plans, so that 



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