244 BRITISH JURASSIC SPONGES. 
between the larger spicules (Pl. XVII, fig. 9c). Tuning-fork spicules are 
present, but it is seldom that they can be recognised in sections. The dermal 
layer consists of three-rayed spicules closely felted together. Some of these, 
weathered out on the surface, are as large as the spicules in the fibres (Pl. XVIII, 
fie. 1a). 
The examples of this species vary considerably in form and size; small 
specimens are 10 mm. in height by 13 mm. in breadth, whilst a large one 
measures 40 mm. in height by 35 mm. in breadth. The smaller forms are 
usually fan-shaped; sometimes the walls are flattened, plate-like, with slightly 
incurved margins; in other instances they are variously contorted, and become 
thickened and lobed. In many of the specimens the dermal layer is no longer 
present, and it is very seldom intact even when it has been preserved, and is 
usually concealed by a growth of small oysters and Serpule. The dermal layer 
appears to be quite distinct from the skeletal fibres, which it covers. 
This species is distinguished from the Neocomian species H. acutimargo, 
Roemer, by its thicker walls, and the oscular apertures are much more closely 
arranged. 
Distribution.—Great Oolite. Hampton Cliffs, near Bath. Woodwardian 
Museum, Cambridge (Walton Collection). Forest Marble at Winsley, near Bath 
(Mr. HE. A. Walford). 
Genus.—Diapuectia, Hinde. 
1883. Cat. Foss. Sponges Brit. Mus., p. 193. 
Syn.—Spongia, Lame. (in part); Pharetrospongia, Zittel (in part). 
Sponges with laminated walls, either cup-, fan-, or platter-shaped. A dermal 
layer is not developed, and there is no canal system apart from the interspaces of 
the fibres. The fibres are in some instances disposed longitudinally, in others 
generally reticulate. The spicular structure consists of axial three-rayed spicules, 
marginal filiform spicules, and tuning-fork spicules, the same as in Hlasmostoma, 
Fromontel. 
This genus differs from Hlasmostoma in the absence of a dermal layer with 
oscules on one surface. This structure is very liable to disappear in the fossil- 
isation, and Prof. v. Zittel has suggested the probability that sponges similar to 
those herein included, in which it is now absent, may have been originally covered 
with this layer. There are, however, numerous well-preserved sponges, such as, 
for example, those of Spongia helvelloides, Lamx., from near Caen, in which not a 
‘trace of dermal layer has yet been noticed, and it seems desirable to include such 
provisionally in a separate genus. 
