PLATE LXX1V. 213 



istic, so also are the spicula of which it is composed ; 

 they are distinctly fusiform but very indistinctly spi- 

 nous at their terminations, requiring a microscopical 

 power of at least 300 linear to define this character in 

 a satisfactory manner. The tension spicula are fre- 

 quently quite as long as those of the skeleton, but very 

 slender, and a power of 400 or 500 linear is required 

 to define their basal spination. The skeleton spicula 

 are stouter than those of the dermal rete, but they are 

 frequently less in length ; the spination of the base is 

 rather abundant and much more strongly produced 

 than that of the remainder of the spiculum, which is 

 sometimes almost destitute of spines, and in all cases 

 they are in a very incipient state, requiring a power of 

 300 or 400 linear to render them distinctly visible. 

 The two forms of retentive spicula are also very minute, 

 requiring a power of not less than 400 linear to define 

 their structures correctly ; the bidentate inequi-ancho- 

 rate form is the most numerous of the two, but both of 

 them are, comparatively speaking, of rare occurrence. 



The only species with which this sponge is likely to 

 be confounded is Halichondria nigricans, but the total 

 absence of bipocillated spicula, and the striking differ- 

 ences in the forms and degrees of spination of the 

 spicula will readily distinguish them. 



HALICHONDBIA AMBIGUA, Bowerlank. 

 Plate LXXIV. 



Sponge parasitical on fuci, firm and fleshy. Surface 

 smooth and even. Oscula simple, large, few in 

 number. Pores inconspicuous. Dermal membrane 

 tough and strong ; abundantly spiculous, reticulated ; 

 rete polyspiculous ; areas square or rhomboidal, occa- 

 sionally with a superadded stratum of dispersed spi- 

 cula; spicula acerate, rather long and stout; of the 

 same form and size as those of the skeleton. Skeleton 

 abundantly spiculous ; rete irregular or subfascicu- 



