26 SILICEOUS SPONGES. 



central prominence. These spicules have only been found detached ; it is probable 

 that they were zone-spicules of one or moi'e species of sponge allied to Thenea. 

 Distribution. Upper Chalk: Horstead, Norfolk ; Kent. 



Genus PACHASTRELLA, 0. Schmidt, 1868. 



Pachastrella prim^va, Zittel. 

 1878. Pachastrella primava, Zittel, Studieu, III Ab. p. 9, t. xi. f. 4a, b. 

 Microscopic slides with detached spicules of this species. Prof. Zittel's collection. 

 Distribution. Upper Chalk : Ahlten ; Hanover. 



Pachastrella convoluta, Hinde, n.sp. (Plate II. figs. 1, 1 a.) 



Sponge growing in plate-like expansions of various forms and dimensions. It is 

 either fan- or ear-shaped, with rounded incurved margins, or folded so that the lateral 

 margins unite to become vasiform, or the walls are irregularly convolute. Some 

 examples are attached to the surface of other sponges ; in others there is no indication 

 of any process by which they were fixed, and these forms may have been free. A 

 large specimen is 110 mm. in length by 100 mm. in width. The walls vary from 

 7 to 12 mm. in thickness. The surface of the sponge is rough, and frequently 

 uneven. The walls are in places penetrated by tubular apertures ; but these are 

 very irregular in size and direction, and appear to be owing rather to extraneous 

 causes than to be of the nature of canals belonging to the sponge. 



The walls are entirely composed of quadrifid sjiicules of various dimensions, loosely 

 mingled together, apparently without definite arrangement, and only held in position 

 by the interlacing of their rays with each other. The spicules of the interior of the 

 walls are only faintly recognizable ; but those of the outer and inner surfaces are 

 better preserved, though even these are considerably altered by fossilization. The 

 rays of the spicules are robust, and apparently obtusely pointed. The length of an 

 arm of what appears to be an average spicule is 0'75 mm. 



This species may be distinguished from Pachastrella primceva, Zitt., by its mode of 

 growth, and also by the dimensions of the spicules, which, so far as I can ascertain, 

 do not reach the size of the larger forms in Zittel's species. In the general form 

 of the spicules, however, and in their disposition this species corresponds with 

 P. primceva, and with the existing forms of the genus, P. abyssi, O. Schmidt, and 

 P. intertexta, Carter*. Detached spicules, apparently resembling those composing 

 this species in form and size, occur in hollow flints at Horstead ; but all the examples 

 in which the form of the sponge is retained are from Flamborough, and appear to be 

 not uncommon in the chalk of that locality. 



Distribution. Upper Chalk : Flamborough, Yorkshire. 



* Ann . & Mag. Nat. Hist. ser. 4, vol. xviii. p. 409, t. xv. f. 41. 



