W. J. SOLLAS ON PHAEETEOSPONGIA STEAHANI. 249 



affin, cut thin slices from it, and mounted them in Canada balsam ; 

 the fibre so prepared exhibited the structure represented in PL XI. 

 fig. 12, which, it will be observed, is exceedingly similar to that 

 figured from Pharetrospongia, PL XI. figs. 5 to 8, of course making 

 due allowance for the fact that fig. 12 is magnified 140, and figs. 5 

 to 7 only 60 diameters. A close examination of the two sponges, the 

 fossil and the recent, fails, indeed, to reveal any essential difference 

 between the structure of their fibre. Minor distinctions of course 

 exist : thus the spicules of the Australian form are 0*0073" long by 

 0*00025" broad, while, as we have already stated, those of Pharetro- 

 spongia are from 0*007" to 0*011" long by 0*00035" broad, and 

 thus are slightly the larger ; the recent spicules (PL XI. fig. 11) 

 are also more abruptly pointed than the fossil ones. 



We have already assigned Pharetrospongia to its famih r , we now 

 go a step farther and place it amongst the Thalyosa, the third group 

 of the Renierida, first because its spicules are all of one kind, next 

 because they are arranged in a fibrous form, and finally because their 

 dimensions are those which distinguish the spicules of the Thalyosa 

 from those of the other allied groups, to which otherwise Pharetro- 

 spongia might belong ; for the same reasons the Australian sponge 

 goes with it and is included in the same group ; and thus the asso- 

 ciation of the two is only separated by one step from generic iden- 

 tity. Here, however, the agreement ceases ; for the Australian 

 sponge possesses a well-marked canal-system, with large and distinct 

 vents ; in this it agrees with the rest of the Thalyosa and differs 

 widely from Pharetrospongia, which, as we have seen, is charac- 

 terized by the absence of clearly specialized oscules and of distinctly 

 defined canals. 



Order HOLOKHAPHIDOTA, Carter. 



Family RENIEEIDA, Carter. 



Group Thalyosa, Carter. 



Genus Phaeeteospoxgia (mihi). 



Description. — A plate-like sponge, variously plicated, contorted, 

 and anastomosing ; growth marginal ; skeleton composed of an irre- 

 gular or vermiculate network, the fibres of which consist almost 

 wholly of acerate spicules lying parallel with each other, with over- 

 lapping ends ; oscules small, oscular surface smooth, with a network 

 of thickened fibres ; poriferous surface rough, undulating, ridged 

 and furrowed, furrows probably exposed intermarginal cavities ; 

 canal-system indefinite. 



Species Phaeeteospongia Steahani (mihi). 



Sponge about one third of an inch thick, other dimensions very 

 variable ; spicules 0*007 to 0*011 of an inch long, by 0*00035 of an 

 inch broad ; fibre 0*006 of an inch in diameter on the average. 



