PLATE LXXIII. 203 



The under surface is traversed by a straight thin stem 

 of, apparently, a zoophyte, so that in its living state it 

 was probably parasitical. The surface on which this 

 stem is situated is nearly flat, and it does not present 

 any very striking characters. The opposite surface 

 which has apparently been the upper one, is slightly 

 rounded, the thickest portion being near the middle. 

 This surface presents the most striking characters. 

 When examined by direct light with a power of fifty 

 linear we observe a number of rather indistinctly 

 defined areas, in each of which are numerous minute 

 pores closely congregated. These little areas are not 

 amenable to an ordinary hand-lens, but by the aid of 

 the power I have named above, they afford prominent 

 and very distinct specific characters. The general 

 structural peculiarities of the skeleton are strikingly 

 different from any others that I have described in the 

 division of the species of Halichondria to which it 

 belongs. The reticular structure of its skeleton is 

 much more regular, and the component spicula are very 

 much smaller than those of any of the nearly allied 

 species, and at the first glance at its anatomy it has, 

 from the regularities of its structure and the smallness 

 of its spicula, very much of the general aspect of an 

 Isodictya ; but on a closer investigation we fail in 

 detecting the slightest indication of the radial structure 



o o 



of the skeletons of the sponges belonging to that genus. 

 The spicula are purely acerate in form, and are but 

 very slightly curved ; an average sized one measured 

 yfg- inch in length and -gifocr inch at its greatest 

 diameter. 



HALICHONDRIA COUCHII, Bowerbank. 

 Plate LXXIII. 



Sponge massive, compressed, sessile. Surface even, 

 smooth. Oscula simple, dispersed, minute. Pores 

 inconspicuous ; dermal membrane pellucid, reticulated ; 

 spicula of the rete same as those of the skeleton ; 



