ISODICTTA. 147 



the skeleton . The spioula, slightly diverging from e^ch 

 other, form a very striking and effective defence to the 

 structures beneath them. 



" The oscula require the aid of a lens of an inch 

 focus to render them distinct to the eye. 



" The dermal membrane is rather sparingly fur- 

 nished with tension-spioula ; but the minute bidentate 

 equi-anchorate retentive spicula are much more numer- 

 ously embedded in the sareode of the membrane, but 

 from their minuteness they may very readily escape 

 observation. Two of the largest measured tsoS i^"^^ 

 and f^ee i^^h in length, and the shafts of the dentes 

 are extremely slender, requiring a power of about 400 

 linear to render them distinct to the eye, in situ, 

 when mounted in Canada balsam. They are not 

 evenly dispersed ; in some parts they are rather few 

 in number while in other parts they occur in much 

 greater numbers. The primary lines of the skeleton 

 consist of from three to five or six spicula in the fas- 

 ciculus, very rarely more. The interstitial membranes 

 are very pellucid; and, although I searched diligently 

 for them, I could not find the slightest indication of 

 either tension or retentive spicula." 



60. ISODICTXA COLLINA, Bow., n. sp., IV, PI. XIV, figs. 

 6—13. 



"Sponge sessile, massive, tumulous. Surface mi- 

 nutely hispid. Oscula slightly elevated and marginated, 

 terminating each tumulus. Pores inconspicuous. Der- 

 mal membrane reticulated ; rete unispiculous, spicula 

 acuate or subfusiformi-acuate, stout, as long as those 



