8 CARBONIKKROUS F014MAT10JSS AND FAI'NAS (IK coLoUADo. 



In a work wliicli lias siitt'cri'd from so rr('(iucMt and so prolonged interruptions 

 iis tins, g'i\'int;' opporliiiuty for many ciiiiiiiics of oj)ini<)n — oii(>, too, in wliicli facts arc 

 iuvolvod so many and so variously rolatetU-I can hardly expect to have escaped 

 errors and inconsistencies. The hope is entertained that they will not detract too 

 much from what value this work would otherwise have had. The task proposed has 

 been found a ditiicult one, appreciation of its magnitude growing as investigations 

 into literature and faunas progressed. It has proved, indeed, that the points in 

 relation to which a demonstration was possible are few, and not, perhaps, very 

 important. The facts are not at hand to detei'mine many of the questions which 

 have arisen at different points of my investigations. I have permitted myself, how- 

 ever, to frame a number of hypotheses, and to suggest a number of possibilities. It 

 would indeed be gratifj'ing to think that in this way the solution of problems 

 whose answers have eluded me has in any^ measure been facilitated. 



It seems desirable to offer a few words of explanation, or it may be of excu.se, for 

 several omissions and commissions in the following pages that may be considered 

 serious enough to excite criticism. One license which I have permitted myself is 

 the equalization, for purposes of convenience and simplicity, of zoological groups 

 whose unequal importance is generally recognized. That the difference is of so 

 general recognition is perhaps the greater excuse for this lax usage, this concession 

 to convenience. 1 refer at present especially to the placing of the Bryozoa and 

 Brachiopoda and the several divisions of the true Mollusca upon a temporary parity 

 with the Protozoa, the sponges, and the other groups having the rank of classes. This 

 arrangement has been observed in the tables and in the discussion of species, and is 

 one which the paleontologist is almost certain to make, mentally at least, in studying 

 a A'aried fauna. It amounts, in short, to breaking up the larger and more unwieldy 

 groups into recognized subdivisions, from which the master titles are omitted. A 

 slightly different practice is that of the constant use of subgeneric names in a titu- 

 larly generic sense. For convenience and brevity, each subgenus has been written 

 free, as if itwere an independent genus: an indulgence from which I would certainly 

 have abstained had this work been planned upon systematic or biologic lines. 



Manv are the names which would be mentioned if all were to be set down to whom 

 recognition is due for assistance, great and small, in the preparation of this paper. 

 Fir.st, perhaps, should be mentioned the geologists, Messrs. Cross, Spencer, Eldridge, 

 Emmons, Lee, and others, through whose accumulation of material ni}' work was 

 not onlyr made possible but even suggested. Acknowledgments are due also to the 

 Museum of Chicago University, from which, through Mr. Stuart Weller, I have 

 enjoyed the privilege of examining and illustrating some specimens importantly 

 connected with my subject. I have consulted freelj^ with my colleagues, Mr. 

 Schuchert, Mr. Ulrich, and Mr. Bassler, upon paleontologia whenever occasion has 



