1887.] NATURAL SCIENCES OF PHILADELrHIA. 181 



is determined by other appendages of the gemmulse. Assuming 

 that the ilhistrations will sufficiently show the meaning of the si)e- 

 cial terms used, the student is referred, without further preface to the 

 following "Key," by com})arison with which he will without 

 hesitation be able to decide the generic status of his specimen. 

 Fuller definitions will be given as each genus comes under no- 

 tice. 



KEY TO THE GENERA OF FRESH WATER SPONGES. Carter's System. 



1. Gemmulse surrounded by acerate (PL VII c.c.c) or cylindrical 



(PI. VII b.b.b) spicules alone. (Plate V, figs, i, ii, 

 \\{\ ^ Spongilla. 



2. " surrounded by birotulate (PI, [X. fig. iii. b.b.b etc.) 



spicules of a single class or type,^ resting by one 

 (the proximal) rotule upon the chitinous coat ; 

 diameters of the rotules equal or nearly so. (PI. 

 V, fig. iv, V and vi.) Meyenia. 



3. " surrounded by birotulate spicules of two classes or 



types, both resting by one rotule upon the chitin- 

 ous coat ; the less numerous class longer than the 

 other. (PI. VI, fig. i,) (PI. XL b.c. etc.) 



Heteromeyenia. 



4. " surrounded by iniequibirotulatespicules(Pl. XII fig. 



i, ii, and iii. b.c.d. etc.) of which the proximal ro- 

 tule is much larger than the distal one. (PI. VI. fig. 



ii.) TUBELLA. 



'5. " whose crust" is charged Avith spicules from which 



the distal rotule has been entirely eliminated, leav- 

 ing the proximal rotule surmounted only by a 

 short, pointed portion of the shaft. (See Ann. and 

 Mag. 1881, PI. 5, figs. 1 and 2.) Parmula. 



i). " whose forarainal tubules are prolonged, their ter- 



minations broadly funnel-shaped or divided into 

 cirrous appendages of varying numbers and 

 lengths. (PI. VI, figs, iii, iv, v and vi.) 



Carterius. 



1 Specimens are occasionally found with birotiilates of a single type arranged 

 in two or three concentric series. For this form Mr. H. Mdls proposed (Proc. 

 Am. Soc. of Microscopists, 1884) the new genus PUioineyenia; while Prof. 

 Vejdovsky has merged two species named on account of the same peculiarity into 

 the common species E. mi'tUeii. See "Diagnosis." 



