RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 353 



Every known member of all the families of the class Echinodermata 

 is a denizen of marine waters. Therefore the discovery in any forma- 

 tion of fossil remains referable to this class is accepted as proof of its 

 marine origin. 



ANNTLOSA. 



The classes Myriapoda and Arachnida arc omitted from this review 

 because they are all land animals except the few that rind a congenial 

 habitat in more or less direct contact with water. 



In their adult state the Insecta are generally land animals, but some of 

 them resort to at least a partially aquatic habitat. In their larval state, 

 however, many insects are true aquatic animals, usually living in fresh 

 waters. In this state some of them possess no such skeletal or protec- 

 tive parts as are likely to resist decomposition, but many of them, like 

 the adult Insecta, Arachnida and Myriapoda, are provided with a chi- 

 tinous covering which retains more or less completely the bodily form 

 of the animal after its death and decomposition. Therefore such forms 

 arc not uufrequeutly found preserved in a fossil state, but because all, 

 or nearly all the members of those three classes had either a land or 

 fresh-water habitat their remains are much more likely to find intomb- 

 ment in the sediments of fresh waters than in those of either brackish 

 or marine waters. Indeed no such remains are known to have been 

 found in any deposits which are unmistakably of marine origin. 



Crustacea. — All the Crustacea being provided with gills or other 

 organs suited to aqueous respiration have a peculiarly direct relation 

 to the subject of these discussions. All of them also being provided 

 with a more or less complete dermal skeleton or outer covering which 

 resists decomposition and retains the form of the animal after its death, 

 necessarily are of great paleontological importance. It is, however, 

 true that their practical value in the present discussion is much less 

 than that of the Mollusca not because of any inferiority, but because 

 crustacean remains are comparatively very rare, especially in all North 

 American strata which represent that portion of geological time during 

 which were deposited the principal unmistakably non marine forma- 

 tions. For this and other reasons a tabulation of the families of the 

 living Crustacea like that of the fishes and mollusks on preceding 

 pages has been omitted. Some of the uiore important facts concerning 

 the range of habitat of certain of the various groups which constitute 

 this class are, however, recorded in the following paragraphs. 



This review of the Crustacea is, therefore, somewhat more general 

 than that of the fishes and mollusks. That is, no regular classification 

 of the Crustacea is attempted, but tin 1 remarks are ranged under the 

 head of the general divisions of the class, and direct reference is made 

 only to those families or other subdivisions representatives of which 

 arc found in either fresh or brackish waters or both. All those sub- 

 divisions, which are not specially mentioned, are not known to live in 

 other than marine waters. 



H. Mis. Ill, pt. L* L'3 



