92 EDWARD POTTS : 



contrast with it the one which is to be my present subject.* 



DISCOVERY AND DESCRIPTION. 



In January, 1885, Microhydra ryderi was discovered under 

 similar conditions to those in which the writer has ever since 

 seen it growing, viz. : lying perdu on the surface of rough 

 stones, under the protection of several species of polyzoa — 

 in Tacony Creek, near Cheltenham, Pa. It was found later 

 in a more favored locality — in the Schuylkill Canal near Flat 

 Rock dam, not far from Shawmont station, on the Schuylkill 

 River. When single headed its cylindrical length was about 

 one-fiftieth of an inch ; its thickness about one-fifth of its 

 length. It was entirely without tentacles, but the free or head 

 end was crowned with a capitiihim bristling with from twenty 

 to fifty lasso or thread cells and terminating in a mouth, with 

 difficulty detected (See Plate I, Figure i). More frequently 

 two branches, terminated by capiU(l(X, were observed connected 

 near the base ; occasionally there were three branches, 

 and once or twice colonies of four have been doubtfully noted. 

 It is mortifying to have to acknowledge that in all these 

 twenty-four years I have never once seen the budding or 

 growth of this second or third stem from near the base of the 

 first, although bicapitate forms are much more frequent than 

 single ones. A branch bearing a head will, however, be 

 mentioned a little later. 



TWO LOCALITIES DESCRIBED. 



Tacon}^ Creek is a shallow mill stream with its shifting 

 bed composed of riffs and shallows, and but few have recently 

 been collected there. Singularly favorable conditions, how- 

 ever, are found at Flat Rock dam, at a place where a contin- 

 uous influx of water is made to feed several mills further 



* Before the latter appeared there had been fomid one other hydroid 

 in fresh water (Cordylophora lacnstris) , but its great dissimilarity from 

 either of the others and its striking general resemblance to manj- of the 

 marine forms, seem to place it in a quite different class. 



