340 A MONOGRAPH OF THE 



cula birotulate, short, rarely spinous; disposed in 

 lines radiating from the centre of the ovarium ; rotulae 

 equal in size, flat, deeply and irregularly dentate, 

 diameter equal to the length of the shaft of the spi- 

 culum. 



Colour. Alive, yellow or green. 



Habitat. Rivers and lakes of Great Britain ; on old 

 timber and branches of wood. 

 Examined. Alive. 



Dr. Johnston, in his c History of British Sponges,' has 

 given so able a digest of all that has been written regarding 

 this species and its numerous changes of name, both generic 

 and specific, as to render any observations on that part of 

 my subject a work of supererogation. 



The normal form of this species is certainly massive and 

 sessile ; and the arborescent form that it is said to occa- 

 sionally assume is due to its having originally been para- 

 sitical on stems of plants, and, perhaps, also not unfre- 

 quently to S. lacustris having been mistaken for this species. 

 When developed under favorable circumstances, I have 

 seen large rounded lobes projected from its surface ; but I 

 have never seen it assume an arborescent form like that of 

 JS. lacustris. 



The absence of spicula in the dermal membrane of this 

 species readily distinguishes it from S. lacustris ; but the 

 most striking differential character exists in the birotulate 

 spicula of the ovaria, the correct form and position of 

 which were, I believe, first pointed out by Meyen in Valen- 

 tin's ' Repertorium,' 1840. 



The shaft of the spiculum is usually smallest at the middle, 

 and it increases more or less as it approaches the rotulae ; 

 and, occasionally, but rarely, we rind a single large spine 

 projecting at right angles from near its middle. The rotulae 

 are flat and deeply and irregularly divided ; the divisions 

 frequently extending from the circumference to very near 

 the centre. They are disposed very closely together in the 

 walls of the ovaria ; the outer rotula supporting the external 



