20 Studies in Geology, No. 3 



In the systematic presentation of this fauna the author has 

 endeavored to supply accurate references to all forms dis- 

 cussed. These references are to original descriptions in all 

 cases excepting those in which subsequent treatment is 

 better either through superiority of figures or greater com- 

 pleteness of description. References have been made to all 

 species with which comparison is pertinent, and although it 

 may seem to some that too large a number of similar forms 

 has been quoted in the remarks on some of the species, the 

 writer has included such as are presented out of first-hand 

 knowledge of the assistance they render to students of 

 similar faunas. Paleontologic work as a whole is character- 

 ized by too great brevity of treatment in matters of detail 

 and comparison — brevity at the expense of efficiency on the 

 part of those who find need later to use the work; and it 

 is certainly for the benefit of such, in greatest measure, that 

 details are mentioned at all. 



Ill — Geoi^ogic. Analysis. 



The determination of the age of a fauna, and particularly 

 one from a region somewhat distant from the nearest well 

 known locality, ordinarily involves a number of uncertain- 

 ties which make advisable, if not necessary, the obtaining of 

 complete data before valuable conclusions may be drawn. 

 This is particularly true of faunas which show affinities suf- 

 ficiently scattered to cause careful weighing of all evidence, 

 and since many fossil assemblages are of such character an 

 attempt at correlating a fraction of a fauna, if advisable, 

 should be accompanied by some statement as to the degree 

 of faith to be placed in the results. It may be said in behalf 

 of the present study that the affinities of the Zorritos fauna, 

 and particularly that of the Lower Zorritos formation, are so 

 strongly centralized that the discovery of the numerous 

 additional species which no doubt exist may confidently be 

 expected to confirm the present deductions, and under no 

 circumstances to change the character of the evidence 



