220 PROCEEDINGS OF THE ACADEMY OF [1887. 



irregular ; often exhibiting the deep grooves of incomplete, concen- 

 trating canals, covered only by the dermal film. Pores and osteoles 

 conspicuous. Texture rather firm ; main lines of skeleton spicules 

 and of canals horizontal or nearly parallel to the base of su2)port. 

 Spicules fasciculated. 



Gemmules numerous throughout the entire sponge, but in the 

 deejjer parts, most abundant ; spherical, very variable in size ; crust 

 variable in thickness; in some forms nearly or quite wanting, 

 whilst in others it is so thick as to promote the formation of birotu- 

 lates in two or three concentric zones. 



Skeleton spicules curved, fusifoiin, gradually pointed; varying 

 even in the same specimen from altogether smooth to entirely spined; 

 but nearly always smooth at the extremities; spines, when present, 

 minute, conical. (PI. IX, figs, i to iv.) 

 Dermal and interstitial spicules wanting. 



Birotulate spicules of the gemmules generally short ; rotules flat, 

 irregularly rayed, occasionally microspined, rays straight, deeply 

 cat, nearly to the centre of the rotules; shafts rarely spinous; spines 

 long, tapering. (PI. IX, figs, i to iv.) 



Meas. Skeleton spicules O'Ol by 0"0005 inches. Length of 

 birotulate 0*0005 inches. Diameter of rotules 0"0007 inches. 

 Hah. In lakes or ponds ; preferring standing water. 

 Loc. Throughout the Eastern and Middle United States gener- 

 ally. 



This species under its former name of Spongilla fluviatilis has long 

 been known as one of the two fresh water sponges which, until recently, 

 were the only ones generally recognized in Europe. Like S. lacus- 

 tris it is cosmopolitan in habit, and the study of it in this country 

 from a great number of localities, has shown that it, also, is very 

 variable in some of its parts. For instance, its skeleton spicules, as 

 stated above, vary, in specimens from different localities, and to a 

 less extent in those from the same locality, or even in the same 

 specimen, from totally smooth to entirely spined ; and the species 

 named on account of the prevalence of one or other of these condi- 

 tions will have to be given up. Its birotulates also vary greatly in 

 size and proportions, in contour of their shafts and the character of 

 their rays. A' few forms may be so definite in their peculiarities, 

 and rej^eat them in so many localities, as to warrant the use of 

 varietal names ; but in this as in other cases it seems best to include 

 all within a definite range in the species under its familiar name. 



