40 A MONOGRAPH OF THE 



From the description given by Dr. Johnston, in page 

 181 of his 'History of British Sponges/ of Grantia fistu- 

 losa, there is little room to doubt that the sponge sent to 

 him by Mr. William Thompson, and those found by my 

 friends Mr. Stewart and Professor Forbes are of the same 

 species ; but from the structure of the latter specimens, it 

 is quite certain that they cannot be considered as Grantias 

 according to the structural limitations of that genus. 



In Mr. Stewart's and Professor Forbes's sponges, there is 

 not the slightest indication of the large, regular, intersti- 

 tial cells that radiate from the central axis of the sponge 

 in G. ciliata or compressa ; although there is a large cen- 

 tral cloaca as in those species. On the contrary, the whole 

 of the walls of the animal closely resemble a halichondra- 

 ceous sponge in structure, and is permeated as in that tribe 

 by numerous contorted canals, the excurrent orifices of which 

 are at the inner surface of the sponge ; which forms one 

 great cloacal cavity as in the true Grantias. I have, there- 

 fore, referred these sponges to the new genus Leuconia. 



The spicula with which the external surface is armed 

 are very stout and strong, but not so very numerous ; they 

 are similar in size and strength to the large fusiformi- 

 acerate ones, which are recumbent, principally in a longitu- 

 dinal direction at the surface of the sponge, but the defen- 

 sive ones are considerably longer of the two. 



The principal part of the skeleton is constructed of the 

 slender equiangular triradiate spicula, and the rectangular 

 triradiate ones are found more especially at the outer sur- 

 face, and also near the mouth of the cloaca, where they are 

 disposed with the long coincident radii most frequently at 

 right angles to the long axis of the sponge, and the angu- 

 lating ray pointing backwards, thus forming a strong but 

 light and elastic interlacing structure. 



The cloaca is very capacious and abundantly armed with 

 spiculatcd equiangular triradiate spicula ; the spicular ray 

 being long, slender, and gradually attenuating, and they 

 are all slightly curved in the direction of the mouth of the 

 cloaca. 



The slender acerate spicula forming the very meager 



