LETTER TO THE SECRETARY. 



Office United States Geological and 



Geographical Survey of the Territories, 



Washington, D. C, January 1, 1879. 



Sir : I have the honor to transmit herewith, for your approval and 

 for publication, Volume XII of the Final Reports of the Survey under my 

 direction. 



This volume has been prepared by Prof. Joseph Leidy, the eminent 

 comparative anatomist and microscopist, who is well known as one of the 

 most valued collaborators of the Survey, and the author of the first volume 

 of the quarto series of Reports, entitled " Contributions to the Extinct Ver- 

 tebrate Fauna of the Western Territories." 



Professor Leidy spent the greater portion of two seasons in the West 

 under the auspices of the Survey. During this time he made a careful 

 exploration of the country about Fort Bridger, Uinta Mountains, and the 

 Salt Lake Basin, in search of the materials for this memoir. 



The use of the microscope in all branches of natural science has 

 become so universal that the publication of the present volume in con- 

 nection with the Survey needs no explanation. It is intended as a guide 

 and aid to students in this new or little-known field of observation. The 

 facility with which these small objects for study can be obtained all around 

 us will render the work still more timely and useful to the students of this 

 country. There are also a number of closely allied fields of inquiry, as the 

 Diatoms, Desmids, Infusoria, Rotifera, Entomostraca, and Aquatic insects, 

 etc., which, when pui'sued with the same skill and devotion, will prove equally 

 fruitful in results. 



The Rhizopods are the lowest and simplest forms of animals, mostly 

 minute, and requiring high power of the microscope to distinguish their 

 structure. While most of them construct shells of great beauty and variety, 



