272 A MONOGRAPH OF THE 



broad, and six lines in thickness ; and when examined in 

 the dried state with a two inch lens, the appearance is 

 much that of a mass of tow wetted, closely compressed, 

 and dried in that state. This appearance is produced by 

 the large size of the reticulations of the dermal membrane, 

 the areas of which are very open and irregular in form, 

 and the rete very stout. The tension spicnla are very 

 abundant in the membranes of the areas, but the retentive 

 spicnla are few in number. The spicula of the rete are 

 compactly fasciculated together. Numerous minute circular 

 nucleated cells were imbedded in the sarcode on the inner 

 surface of the dermal membrane. The reticulations of 

 the skeleton are strongly and compactly constructed, but 

 not to quite so great an extent as in the dermal membrane. 

 The profusion and large size of the retentive spicula of the 

 interstitial membranes frequently give the whole of that 

 portion of the skeleton in the field of view the aspect of a 

 Hymeniacidon, but the definite fasciculation of the skeleton 

 in other parts corrects that illusion. 



The retentive spicula appear to be more abundant in the 

 interstitial membranes than in the dermal one, and the nu- 

 cleated cells are as numerous in the interstitial membranes 

 as in the dermal membrane. 



On the side of this specimen is firmly imbedded the 

 type specimen of Halichondria simplex ; both having lo- 

 cated themselves on the same Zoophyte, a Tubularia ; they 

 have been closely pressed together by juxtaposition and 

 progressive development. It is a singular accident that 

 two new species should thus be found conjoined. 



