344 A MONOGRAPH OF THE 



the terminations ; while in the former the spicula are stouter 

 and shorter in their proportions, and the attenuations com- 

 mence at or very near the middle of the shaft of the spi- 

 eulum, and are, therefore, fusiformi-acerate in shape. 



But the greatest organic difference between this species 

 and S.fluviatilis exists in the spicula of the ovaries. In 

 the latter, the case of that organ is strengthened and sup- 

 ported by a number of birotulate spicula, the rotulae sup- 

 porting the inner and outer surfaces of the case of the 

 ovary, the shafts of the spicula being disposed at right 

 angles to the surfaces ; while in 8. fluviaUlis the walls of 

 the ovary are totally destitute of birotulate spicula, but in 

 their place we find a considerable number of curved, acerate, 

 spinous spicula, not disposed at right angles to the sur- 

 face of the gemmule, but imbedded in and lying parallel 

 to the surface of that organ, thus affording a specific dis- 

 tinction so strikingly different from the corresponding struc- 

 tures in S.fluviatilis as to render the discrimination of the 

 species easy and certain whenever the ovaries are present. 



This species occurs plentifully at the bottom of the West 

 Country Timber-dock, on the south side of the Thames, 

 near Rotherhithe. It may frequently be found attached to 

 the lower part of the large mooring-posts near ths central 

 parts of the docks, about eight or ten feet deep. I have 

 never found it in this locality in shallow water or near the 

 surface like 8. fluviatilis,yf\x\.Ghi^ also abundant in the same 

 dock attached to the floating timber. On the contrary, at 

 Cookham, a few miles beyond Maidenhead, this species is 

 abundant on the posts and sides of the wharfing-boards ; 

 and here it is always found near to the surface of the water, 

 and has a very fine emerald-green colour. 



