1V LETTER TO THE SECRETARY. 



their soft part consists of a jelly-like substance. This the animal has the 

 power of extending in threads or finger-like processes, which are used as 

 organs of locomotion and prehension, often branching. From the appear- 

 ance of their temporary organs, resembling roots, the class of animals has 

 received its name of Rhizopoda, meaning literally root-footed. 



In compensation for the smallness of these creatures, they make up in 

 numbers, and it is questionable whether any other class of animals exceeds 

 them in importance in the economy of nature. Geological evidence sliows 

 that they were the starting-point of animal life in time, and their agency 

 in rock-making has not been exceeded by later higher and more visible 

 forms. 



With the marine kind, known as Foraminifera, we have been longest 

 familiar. Their beautiful many-chambered shells — for the most part just 

 visible to the naked eye — form a large portion of the ocean-mud and the 

 sands of the ocean-shore. Shells of Foraminifera likewise form the basis 

 of miles of strata of limestone, such as the chalk of England and the lime- 

 stones of which Paris and the pyramids of Egypt are built. 



Fresh-water Rhizopods, though not so abundant as marine forms, are 

 nevertheless very numerous. They mainly inhabit our lakes, ponds, and 

 standing waters, but they also swarm in sphagnous swamps and even live 

 in moist earth. Professor Leidy has devoted several years of study to 

 the Fresh-water Rhizopods of the eastern portion of our country, and his 

 especial object in his western expeditions was to investigate those which are 

 to be found in the elevated regions of the Rocky Mountains. 



The beautiful plates which illustrate this volume were engraved by 

 Messrs Sinclair & Son, Philadelphia, and to this firm the thanks of the Sur- 

 vey are due for their care -and skill. 

 Very respectfully, 



F. V. ITAYDEN, 



United States Geologist. 



To the Secretary of the Interior. 



