PLATE LXXXV. 287 



springs from a thin base not exceeding a line in length 

 and about half a line in width. When examined in 

 water the whole of the sponge was nearly opaque, and 

 it was not until it was mounted in Canada balsam that 

 its structural character could be determined. 



It is nearly of the same diameter from its base to its 

 apex. There are no indications of hispidation on any 

 part of either the base or the ascending portion of the 

 sponge. The dermal membrane closely invests the 

 sponge for about two thirds of its length upwards from 

 the base ; but that portion of the column is so opaque 

 that the spiculation of its dermal membrane is but very 

 indistinctly to be seen ; but at a portion of the thin 

 margin of the base its characters were distinctly 

 demonstrable ; and the spicula were there seen to be 

 closely packed together either in broad parallel groups or 

 irregularly dispersed. They are of the same form as 

 those of the skeleton, but rather less in length and 

 diameter. 



At a few small spots on the column they were also 

 rather indistinctly visible, and they appeared to be 

 there disposed in parallel groups in lines coinciding 

 with the long axis of the sponge. The primary lines 

 of the skeleton vary from two to three or four spicula 

 in thickness, and they all run parallel to each other in 

 accordance with the long axis of the sponge. The 

 secondary lines of the skeleton appear all to be uni- 

 spiculous, and the interstices of the rete are most 

 frequently square. 



From the general appearance of the structures of 

 the sponge, it appears to be in an adult state, and if 

 the specimen under consideration be of its usual size it 

 will be readily distinguished from its nearest allies in 

 the group of species to which it belongs. There are 

 only two species with which it is likely to be con- 

 founded Isodictya ramusculus and I. clava. 



The former has the skeleton spicula shorter but 

 much stouter than those of I. filamenta, and it differs 

 also in the unispiculous structure of the skeleton. In 



