LETTER OF TRANSMISSION. 



United States Engineer Office, 

 Geographical Explorations and Surveys 



West of the One Hundredth Meridian, 



Washington, D. C, June 10, 1875. 



Sir : I have the honor herewith to transmit my final report upon the 

 invertebrate fossils collected by the exploring parties under your command 

 during the years 1871, 1872, and 1873* 



The collections were placed in my hands in July, 1874,* and I at onee 

 commenced a critical examination of them, followed by the work of pre- 

 paring the whole for publication; which work I have continued uninterrupt- 

 edly to the present time. A preliminary investigation of these collections 

 showed that quite a large proportion of the species were new to science, 

 and, for the purpose of securing to your expedition due credit for priority of 

 discovery, I wrote out descriptions of the new species then recognized, and 

 sent them to your office, with the request that they be immediately published, 

 for the purpose of securing the object named. This publication was made 

 from the Grovernment Printing-Office in December, 1874, and is the one 

 referred to in the synonymy accompanying a republication of those species 

 in this final report, together with additions and illustrations. Another object 

 of this preliminary publication was the announcement of important geological 

 conclusions from paleontological evidence. Among the most interesting of 

 these may be mentioned the distinct recognition of the fauna of the Canadian 

 period in Nevada and Utah ; that of the Trenton period in Utah and New 

 Mexico ; and that of the Subcarboniferous period in Arizona, Nevada, and 

 Utah. 



Besides the new species described in the preliminary report, a full 



* Collections of 1874 have since been placed in my hands for investigation, and the results have 

 been incorporated into the following report. 



