Genus of Fossil Hexactinellid Sponyes, 5 



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

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

 riiis arrangement accords with the general rule, that in all 

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

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

 Hurt'ace 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 tliis 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 

 mucii diminished in thickness ; we then see the oscules freely 

 exposed over the denuded area, and dimly to be discerned 

 througlx 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. II. 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 

 (PI. I. figs. 2, 4, 6, 7, 8, PI. II. figs. I & 2), which, 

 unlike the oscular plate, rapidly increases in thickness from 

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



