PLATE LXXV. 221 



tliey are so minute and delicate in their structure that 

 they require a microscopical power of 1000 linear to 

 define them in a satisfactory manner. The two forms 

 are irregularly dispersed in about equal proportions 

 on the inner surface of the dermal, and on both sur- 

 faces of the interstitial membranes. This sponge is 

 the second case of the occurrence of the siuglar form 

 of retentive spiculum, the bicalcarate bihamate one. 



When we compare the specific description of IsotUctya 

 Normani with the species under consideration we 

 observe a considerable amount of similarity in their 

 organic structures, but although their organs closely 

 approach each other in fonn they difier widely in their 

 proportions. Thus the skeleton spicula of I. Normani 

 are always more or less fusiform, and are not more 

 than half the length and diameter of the purely acuate 

 ones of I. laciniosa, while the proportions of the bical- 

 carate bihamate spicula in the two species are the 

 reverse of those of the skeleton ones, those of I. Nor- 

 mani being the larger of the two species. Two full- 

 sized spicula of I. Normaiti measured -rnfy and -i~^f2 

 inch in length ; but the differences in their external 

 form will always readily separate the one from the 

 other when in an adult state. 



