14 PREFACE. 



character. The horizons are designated by the larger units of 

 the several groups. As already stated, some of the collections, 

 especially those made by members of the present Survey, have 

 been carefully and accurately labeled with particular reference 

 to the different beds of a detailed section of each locality. 

 When these now isolated sections are properly correlated, as 

 they will be as the work of the Survey goes on, much valuable 

 material will be in hand for broad studies in the distribution of 

 the different faunas. This investigation has already begun, and 

 very important results are foreshadowed. 



Brief nominal histories have been appended to the descrip- 

 tions of many of the most important species, together with 

 some of the most salient points brought out in the present 

 investigation concerning the structural features of the various 

 types. 



In regard to illustration, the leading Missouri species of 

 each genus has been figured, and also some of those forms 

 heretofore described from the State, but never illustrated. 



Little attempt has been made to deal, to any great extent, 

 with the numerous and complex questions of synonymy — not, 

 however, for reason of any inappreciation or under-estimation 

 of their full significance and importance, but on account of an 

 entire inappropriateness, in a publication of this character, of 

 such necessarily prolix discussions. That there exists a bur- 

 densome and extensive synonymy in many of the zoological 

 groups is only too well known to every student of ancient life ; 

 in fact, it is so manifest to everyone who has given the subject 

 even a casual consideration, as to at once render apparent the 

 cogent necessity of a careful and complete revision of most of 

 the sections. The wide geographical distribution of some 

 forms, and the concomitant changes of environment, may be 

 referred to as among the chief causes of local variation of 

 species. Notwithstanding the painstaking and conscientious 

 labors of some of the earlier American writers on the subject, 

 the question of specific range in time and space did not receive 

 the attention in the beginning that it did subsequently, and there- 



