258 REPORT OF NATIONAL MUSEUM, 1892. 



likely to have become quite completely represented by fossil remains, 

 because their crust aceous coverings would have found ready intomb- 

 ment in the sediments of the waters in which they lived. It is never- 

 theless true that while crustacean remains arc by no means rare in 

 paleozoic strata, in mesozoic and later formations remains of this class 

 have rarely or never been found abundant, and often they do not appear 

 among fossil faunas, the members of which would seem to have been 

 their natural associates. This is all the more noteworthy because of 

 the frequent molting of the mature shell by these animals, each indi- 

 vidual of which would thus produce many fossilizable counterparts of 

 itself. 



The Myriapoda, Arachnida, and lusecta are so generally dwellers 

 upon the land, that, as a rule, their remains can reach sedimentary 

 iutombment only by such accidental means as have been mentioned in 

 preceding paragraphs when referring to other land animals. The pres- 

 ervation of such remains of these Annulosa, however, as may undergo 

 sedimentary intombmeut is favored by the fact that they are generally 

 provided with a covering of chitine, a substance which resists decom- 

 position more effectually than do most other hard parts of animals, not 

 excepting bones, teeth, and calcareous shells. 



These three classes, especially the Insecta, arc now represented by 

 myriads of mostly minute animals presenting the greatest diversity of 

 form and of habits of life. It cannot be doubted that at least the 

 Insecta were abundantly represented among the faunas of former geo- 

 logical periods, although fossil remains of them are comparatively rare. 

 This rarity is doubtless due to the fact already indicated that their 

 remains could have reached sedimentary intombmeut only by acciden- 

 tal means, and also in part to the fragile character of the chitinous cov- 

 ering of a large proportion of them. In short, it seems necessary to 

 conclude that comparatively little can ever be known concerning the 

 probably great abundance of Insect, Arachnid, and Myriapodal life of 

 former geological time. 



Only the aquatic Annelida need be considered in this connection, 

 because no satisfactory remains of extinct representatives of the others 

 are likely to be found among any of the fossil faunas. * Even the aquatic 

 Annelida are of less importance as regards the subject of this essay 

 than arc most of the other classes of animals, because, with the ex- 

 ception of the Tubieola, few of them possess such hard parts as 

 instructively represent their different forms after the death of the ani- 

 mal. It is true that some of these arc provided with a more or less 

 delicate chitinous covering which sometimes approximately preserves 

 the form of the animal after the decomposition of the soft parts, and 

 some of them also possess minute teeth. Traces of forms similar to 

 these are sometimes discovered in stratified rocks, as are also such 

 minute teeth as compare with those of some living annelids. 



The living Tubieola, however, secrete an external shell, usually cab 



