(xenus of Fossil HexactinelUd Sponges. 5 



which mark, as we have seen, the posterior face, and which 

 probably served as the special pore-areas of the sponge. 

 This arrangement accords with the general rule, that in all 

 cup-shaped and curved fan-shaped sponges the oscules are 

 placed on the interior surface of the cup or on the concave 

 surface of the fan, while the pore-areas occupy the outer or 

 convex surface in each case. ■ 



The restriction of the oscules to the free part of the anterior 

 plate is only to be seen in tolerably perfect specimens ; in those 

 which are at all worn or much weathered the oscules are 

 exposed all over the anterior surface, and by no means con- 

 fined to its freely projecting part. The absence in this case 

 of the smooth face below, and the appearance of oscular mark- 

 ings in its stead, is evidently the result of attrition, and sug- 

 gests that beneath the smooth surface of unworn specimens 

 the oscules may still exist, but concealed by a superficial 

 coating : a slight examination will set this beyond doubt. In 

 some instances a small patch of the outer coating has been 

 completely worn away, while the rest of it has simply been 

 much diminished in thickness ; we then see the oscules freely 

 exposed over the denuded area, and dimly to be discerned 

 through the thin coating which remains : in perfect specimens 

 the smooth surface may be removed by dissolving the calca- 

 reous matrix of the fossil with acid, and brushing away the 

 superficial network which remains behind ; the oscules are 

 then clearly revealed ; while, finally, if a section be made 

 across the plate, the tubes which lead directly away from the 

 oscules will be seen traversing it at right angles to the exte- 

 rior coating (PI, I. fig. 2, e', and PI. 11. fig. 1, o, fig. 2, o). 



The anterior plate thus possesses the same essential struc- 

 ture throughout ; it is a thin plate perforated completely by a 

 number of parallel cylindrical tubes or excurrent canals, which 

 traverse it at right angles and terminate in front in oscular 

 pits, and behind in pore-areas. Its projection past the poste- 

 rior protuberance shows that it is the first formed of the two 

 structures ; and it would- appear that as it extended itself ver- 

 tically and laterally the posterior mass followed after it for 

 some distance as an aftergrowth, while at the same time a 

 superficial covering coated it correspondingly in front, conceal- 

 ing the oscules beneath, perhaps converting them into pore- 

 areas, and leaving patent those only on the projecting part 

 above. 



Posterior Mass. — The posterior part forms a compact mass 

 (PL I. figs. 2, 4, 6, 7, 8, PI. 11. figs. 1 & 2), which, 

 unlike the oscular plate, rapidly increases in thickness from 

 below upwards and from its edges to the middle of its face ; so 



