S/Jonge.1 fr )i>i Fuaafuti. 317 



from the broken oml <.i\' tlie stalk to the centre of the ui)p;r 

 surface of the disc is K? nun. 



" A smooth imperforate cortical layer covers the outor 

 surface of the stalk an 1 under surface of the disc. . . . Th'i 

 upper surface of the disc is in part pn-f jrate 1 by pore?. 



" There appears to iiave boen an interruption of the growth 

 of the specimen after the formation of the disc, and the later 

 expansion has taken place not uniformly, but only from pirti 

 of its upper surface. At these parts rounded bosses of diffe- 

 rent sizes have been formed, and they, like the orii^inal disc, 

 are perforated by pores on their convex upper surfaces and 

 covered on the sides by an imperforate cortical layer. . . . the 

 greater mass of tiie new growtii appears to have b^en formed 

 by the fusion of three originally distinct bosses. 



" The upper surface, whether of the original disc or of the 

 later formed bosses, is perforated by closely-set pores. Tiiese 

 are sometimes isolated, but often they open into curving and 

 branching grooves, recalling those of the coral Mexadrina. 



"At seven places on the surface of the later growth, and 

 at one on the original growth, the lines of pores are seen to 

 be disposed in a radiating manner about so many centres. 

 The pores along these lines are large, being at least twice the 

 diamt'ter of those distributed elsewhere. 



" The fully formed skeleton is built up of a solid mass of 

 polyhedral elements (-40 to 150 /x in diameter), whose surfaces 

 are united closely together to the complete exclusion of the 

 soft parts." 



For a full account of the structure of the skeleton and soft 

 parts of this sponge reference must be made to Mr. Lister's 

 memoir (16). The skeletal structure of Astrosclera differs so 

 entirely Irom tiiat of other sponges that it will probably have 

 to be j)laced in a third order of Calcarea, the other two being 

 Lithonina and Dialytina ; but Mr. Lister would defer the 

 establishment of such a division until more material becomes 

 available for investigation. 



Pli:ctuumnia, llinde (12, p. 51). 



Plectroninia Hindei, sp. n. (PI. XIIL tig. 1, a~r.) 



The specimen, which forms a minute oval crust on a coral- 

 line, is 4x3 millim. in area and 1 millim. in thickness at the 

 centre, but thinning away to the edges. 'I'he surface appears 

 vitreous, with a faint yellow tinge, the interior being crystal- 

 line vitreous. The crust, which was easily flaked off from 

 tiie coralline, showed a granular crystalline basal layer ; on 



23* 



