68 MONOGRAPH OF THE FRESH WATER III. 



NOTE. 



SINCE the foregoing pages have been put to press, a new fact touching the 

 geographical distribution of Cotius meridional!*, has come to our knowledge, which 

 is deemed of sufficient interest to be placed on immediate record here. This spe- 

 cies has been found in one of the lower tributaries of the Potomac, in Rock Creek, 

 Washington (D. C.). Consequently C. meridionalis occurs in the same hydro- 

 graphical basin as C. visooeus. 



It is well known that the aquatic fauna of the Southern States is very different, 

 both from that of the Middle and that of the Northern. There are comparatively 

 few species which occur throughout the Atlantic States of the Union. Still, faunas 

 and floras, although circumscribed within particular provinces or districts, cannot 

 be defined in their boundaries by mathematical lines, and we frequently find dis- 

 tricts whose lines of deraarkation overlap or interdigitate. 



Now the State of Maryland seems to be placed on the limit, between the faunas 

 of the Southern and Middle States, a fact which will make the study of its natural 

 productions much more difficult, but at the same time of more than ordinary 

 interest. 



The locality, Rock Creek, Washington (D. C.), given at page 53 to C. vi#cous, 

 does not belong to the latter species, to which it was temporarily attributed before 

 an examination of the specimens could be made critically. After this was effected, 

 an oversight has allowed it to remain there. 



