2 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 75 



where they are widely distributed, but, generally speaking, the sym- 

 bols represent definite locations where collections have been made. 



It may be interesting to note that the species of Diaptonius have 

 few variable characteristics. They are morphologically very dis- 

 tinct and maintain this distinctness even when widely separated geo- 

 graphically. Systematists have had little excuse for making varie- 

 ties of the species of this genus. 



It is noticeable, too, that North America has a DiajAoirms fauna 

 quite distinct from that of the other continents. D. hacilUfer^ which, 

 curiously, has been found in only two localities in America is a 

 common species in Europe and Asia. A variety of D. eiseni is said 

 to have been found in Siberia. D. castor occurs in Greenland. With 

 the exception of these three species all the North American Diaptomi 

 are peculiar to the Western Hemisphere, and only one North Amer- 

 ican species has been found in South America. 



Without doubt the principal controlling factor in distribution is 

 temperature; while some species can live under a rather wide range 

 of temperature, many of them are distinctly stenothermal. 



For the convenience of those caring to make determinations of 

 species a key is given which includes all the species now known. 

 This key is based on the similar key prepared by the author for 

 Ward and Whipple's Fresh-Water Biology with some corrections 

 and additions. 



Each species is followed by the date of original publication in 

 order to facilitate reference to the bibliography which accompanies 

 the paper. 



DIAGNOSTIC KEY TO THE KNOWN NORTH AMERICAN SPECIES OF DIAPTOMUS 



1 (20). Antepenultimate segment of the male right antenna without distinct 



appendage 2. 



2 (3). Right and left fifth feet of male nearly equal in length, terminal hook of 



right foot symmetrical Diaptomus oregonensis Lilljeborg, 1889. 



3 (2). Left fifth foot of male shorter than right 4. 



4 (9). Left fifth foot of male reaching beyond first segment of right exopo- 



dite 5. 



.5 (8). Right endopodite of male fifth foot equal in length to first segment of 

 exopodite 6. 



6 (7). Terminal hook of second segment of right exopodite denticulate. 



Diaptomus marshi Juday, 1914. 



7 (6). Terminal hook of exopodite of right fifth foot of male uniangular. 



Diaptomus reighardi Marsh, 1895. 



8 (5). Endopodite of right fifth foot of male longer than first segment of exo- 



podite, terminal hook of right foot biangular. 



Diaptomus mississippiensis Marsh, 1894. 



9 (4). Left fifth foot of male reaching end of first segment of right exopodite 



or only slightly exceeding it 10. 



10 (13). Antepenultimate segment of right antenna of male produced at distal 



end into a blunt point 11. 



