OF THE SPONGIAD^E. 165 



stitial structure of either of the first two, nor the simple 

 fistulose form of the latter one ; but with the exception of 

 the form of the spicula, it closely simulates the structural 

 character of the siliceous genus Halichondria, while it is 

 allied with the before-named calcareous sponges by the 

 possession of cloacae. In consequence of these marked 

 differences in the structure of the skeleton, I have separated 

 it from Grantia as defined by Dr. Fleming, and constituted 

 it a genus, adopting the term Leuconia, which was pro- 

 posed by Dr. Grant as a general designation of the whole 

 tribe of calcareous sponges. Figs. 351, 352, Plate 

 XXVII. 



LEUCOGYPSIA, Bowerbank. 



Sponge. Massive, without cloacae ; formed of irregularly 

 disposed membranous tissues and spicula. Oscula at 

 the external surface. 



Type, Leucoyypsia Gossci, Bowerbank. 



The sponges of this genus are still further removed in 

 structural character from the higher organized genera of 

 calcareous sponges Grantia and Leucosolcnia than the genus 

 Lcucoma is. In the arrangement of the interstitial mem- 

 branes, and the mode of dispersion on them of the skeleton 

 spicula, there is a manifest similitude to the structural 

 peculiarities of the genus Hymeniacidon among the Silicese, 

 and we find a corresponding simplicity in the characters of 

 the spicula in Lcucoyypsia, the type of this genus. There 

 are no regularly determined cloaca? projected from the 

 surface as in Lcuconia, and the excurrent canals of the 

 sponge merge in each other, until they unite in one large 

 canal immediately beneath the osculum, in the manner 

 generally prevailing in the great mass of Ilalichondroid 

 sponges. These large canals have defensive spicula similar 

 in structure to those of the other genera of calcareous 

 sponges. The only known British species of this genus is 

 L. Gossei, Bowerbank ; but I am acquainted with an exotic 



