90 BIOLOGICAL RELATIONS 



with observations made in the future, into a suitable statement 

 of the facts as known. 



In the consideration of a large assemblage of organic re- 

 mains, it becomes necessary at the outset to formulate some 

 plan by which ready reference at all times can be made to the 

 various species. In many reports on the paleontology of states 

 or districts, the systematic arrangement of the fossils treated 

 is in accordance with the geological horizons — the oldest com- 

 ing first and the youngest last. But it is readily seen there 

 are serious objections to this method, aside from its not being 

 adapted to the practical wants of those into whose hands the 

 reports chiefly fall. For these reasons, the plan adopted here 

 is to bring as near together as possible all similar forms in 

 order of their genetic relationships. The classification is essen- 

 tially, then, a zoological one. 



For reasons already stated, a chapter has been devoted to a 

 synopsis of the invertebrate animals generally. In the main it 

 follows Nicholson's tabular view, as given in the recently issued 

 Manual of Paleontology. There are, however, a number of 

 important changes in classification, some abridgments, and a 

 few additions. 



The groups unrepresented in Missouri, so far as is known 

 at least, and those which are not preserved as fossils, are marked 

 with an asterisk. With many of these, though large groups, the 

 subordinate divisions are omitted entirely. In the case of the 

 others, or those found within the limits of the state, the major- 

 ity or all of the minor subdivisions are given, in order to facili- 

 tate any comparisons that may be instituted. Many of the 

 representative genera are also appended to each family. For 

 further details, reference to the subsequent chapters must be 

 made. 



