OF WASHINGTON. 151 



For the past two years I have had in my friend, Dr. Wm. 

 H. Fox, of this city, a valuable colleague in Arachnology, 

 who has, by untiring and systematic labors, acquired quite an 

 extensive collection of Araneae, although he has confined his 

 hunting grounds almost exclusively to a small area in the 

 Rock Creek valley, and I owe to this gentleman some valuable 

 additions to my own material in compiling this paper. 



Quite lately another Arachnologist, Mr. N. Banks, has to 

 be added to the force which explores the Spider-fauna of the 

 District. 



There exist two principal conditions by which the territory 

 of the District of Columbia has become particularly favorable 

 for a rich representation of insects and Arachnida. These 

 conditions are its geographical situation and its peculiar 

 topography. 



The District is located at one of those interdigitations of the 

 two great faunal zones of our continent the Boreal and the 

 Sonoran, or, to be more definite, of the most easterly of the 

 boreal branches, the Alleghanian, and that sub-region of the 

 Sonoran zone, known as the I^ouisiauian or Austroriparian 

 branch. Both of these regions meet and intermingle here, 

 and we find here, therefore, representatives of the Arachnidan 

 fauna of both zones. 



The second favorable condition we find in the diverse and 

 peculiar elements of its topography. A large river the 

 Potomac flows through its entire length from north to south, 

 coming from the mountains of West Virginia and western 

 Maryland, and bringing many forms to us which otherwise 

 would hardly be found so far south ; on the other hand, by its 

 course into the Chesapeake Bay, this great waterway opens 

 the road for many southern species. 



The large tributary of the Potomac, the Kastern Branch, 

 with the almost stagnant waters at its lower course, only 

 moved by the ebb and tide, with its swampy and, for the most 

 part, uncultivated shore, offers an excellent breeding place 

 and habitation for palustral genera. 



Another tributary in the District the Rock Creek, with 

 its many small branches, by its course through hill and dale, 



