228 PLATE LXXVI. 



areas of tliis tissue the retentive spicula were distinctly 

 to be seen in situ by the aid of a power of about 500 

 linear. Tliey are very minute, and when separated by 

 boiling in nitric acid and mounted in Canada balsam 

 they require a microscopical power of 700 or 800 linear 

 to render them distinctly to the eye. 



The skeleton is very irregular in its structure ; the 

 primary linear are often very tortuous, and the second- 

 ary ones have frequently as many spicula in them as 

 the primary ones, while at other times they are diffused 

 in a series of single parallel spicula closely adjoining 

 each other ; so that unless a section of the sponge be 

 made carefully at right angles to its surface, a very 

 confused and unsatisfactory view of its structures mil 

 be obtained. The spicula of the skeleton vary con- 

 siderably in their proportion as regards length, and 

 many of them exhibit a tendency to be flecto-acuate. 

 The interstitial membranes are sparingly fmmished 

 with tension spicula of the same form as those of the 

 skeleton, and the retentive spicula are of very rare 

 occurrence. 



The nearest structural alliance to this sponge in 

 the genus Isodictya is I. uniforms, with which it agrees 

 very closely in its skeleton structure and in the form 

 of its spicula, but in their projoortions they differ 

 slightly. In /. mnformis they are rather longer, being 

 as five to four to those of /. imitata. The two sjjonges 

 differ also in external characters ; I. uniformis when 

 dried being of a cream white, while I. imitata is brown, 

 with a tint of green. The presence also in the latter 

 of bidentate, equianchorate, retentive spicula in the 

 dermal membrane ; and the absence of tension spicula in 

 that organ also mark the distinction between the two 

 species in a satisfactory manner. 



IsoDiGTi"A CGiJiACEA, Bowerhaiil: 

 Plate LXXAa. 

 Sponge coating. Surflico even, minutely hispid. 



