ASTROSCLERA WILLEYANA, THE TYPE OF A NEW FAMILY OF SPONGES. 473 



(g) To these points of difference is perhaps to be added the character of the 

 flagellated cells lining the ciliated chambers. They appear, though the point needs 

 confirmation from specially preserved specimens, not to be collar cells of the ordinary 

 type, but more or less amoeboid and without a collar, and with one of their processes 

 gradually tapering into the flagellum. 



From the Calcarea Astrosclei'ci also differs in the following features : — 



(a) The flagellate cells are limited to about half the interior of the ciliated 

 chambers. 



(b) There appears to be a long and complex canal system both on the afferent 

 and efferent sides of the ciliated chambers. 



(c) The mineral constituent of the skeleton is arar/onite, not calcite. 



Comparison of Astrosclera 7vith the Pharetrones. 



The Pharetrones are calcareous sponges which are found in beds ranging from 

 the Carboniferous to the Cretaceous formations. In the arrangement of their skeleton 

 they differ widely from living sponges except Petrostronia the type of the Lithonina. 

 Dr Hinde has recently described' some sponges from the Eocene beds of Australia 

 which appear to be intermediate forms connecting the Lithonina and the Pharetrones. 



The resemblance in general characters between Astrosclera and some of the 

 Plmretrones appears to me so striking as to deserve consideration, although I have 

 to conclude that the differences in the minute structure of the skeleton forbid their 

 being considered as really allied. 



The feature characteristic of many members of this group (though authorities are 

 by no means agreed on its essential cliaracters or limits) is that the spicules are united 

 in close apposition to form trabeculae or ' fibres,' as they are technically called. Dr 

 G. J. Hinde writes, " in no existing sponge is there the same disposition of the spicules 

 of the interior to form anastomosing solid fibres as in this and other allied fossil 

 forms ; and this character forms the distinctive feature of the Pharetrones, sharply 

 marking them off from the families of existing Calcisponges."' 



The centre of the sponge is often occupied by a space, the gastral cavity, with 

 a wide opening above, and penetrating more or less deeply into the interior. The 

 system of anastomosing canals which lies between the trabeculae of the skeleton 

 communicates on the one hand by fine pores with the exterior, and on the other, 

 either by irregularly anastomosing channels or by definite tributary trunks, with the 

 gastral cavity. In some cases the gastral cavity may be so shallow as almost or 

 altogether to cease to exist, and in that case a group of large tributary trunks opens 

 directly to the exterior or at the bottom of a shallow depression. A cortical layer, which 

 has been thought to be imperforate, often clothes the sides of those members of the 

 group which are pedicellate or cylindrical, and if this is so the pores opening to the 



1 G. J. Hinde, " Calcisponges from the Eocene of Victoria {.Australia)." Quart. Joiirn. Geol. Soc, Vol. 

 LTi. 1900, p. 50. 



» "Notes on Fossil Calcispongiae, " .Inn. and .Mag. of Nat. Hist. Ser. 5, Vol. x. (1882), p. 190. This 

 was written before the discovery of the Lithonina. 



