ISCHADITES. 121 
1875. Iscuaprres K@enrau, Giimbel. Beitrige Abhandl. k. bayer. Akad. Wiss., 
Bd. xii, Abtheil. i, p. 43, pl. a, figs. 28, 
29, 30. 
1878. — — Nicholson and Etheridge, jun. Silur. Foss. Girvan, 
p- 20. 
1878. a — Quenstedt. Petref. Deutschl., vol. v, p. 592. 
1880. — — Zittel. Wandb. der Pal., vol. 1, p. 728. 
1880. — — F, Roemer. Lethza Pal., p. 291. 
1882. — — Rupert Jones. Cat. Foss. Foram. Brit. Mus., p. 2. 
1884. — — Hinde. Quart. Journ. Geol. Soe., vol. xl, p. 836, 
pl. xxxvi, figs. 1, 1a—o. 
Sponges depressed-conical, or ovate in form, with convex, flattened, or rarely 
concave bases, which are quite smooth, and do not show any indications of a stem 
or surface of attachment. The summits are generally truncate or evenly rounded, 
occasionally with a slightly-elevated neck ; the central aperture is from 2 to 5 mm. 
in width. Small individuals are only 4 mm. in height by 6°5 mm. in greatest 
width, whilst large forms are 40 mm. by 45 mm. 
At the basal nucleus or commencement of growth there are eight minute 
spicules, with diamond-shaped summit-plates, disposed in a stellate form; the 
spicules succeeding these gradually increase in size to the zonal region of the 
Sponge, and then gradually diminish again towards the summit. The spicular 
summit-plates in the zonal areas vary in different specimens from 2 to 4 mm. in 
width, whilst near the top of the Sponge surrounding the aperture they are only 
from ‘25 to *4 mm. wide. The four horizontal or transverse rays of the spicules 
as a rule slightly exceed in length the semi-diameter of the respective summit- 
plates, and thus overlap each other. In some cases the transverse ray, which 
points towards the top of the Sponge, extends beyond the summit-plate, and 
projects partly over the plate of the spicule next above it. The spicular rays 
extending inwardly towards the centre of the Sponge vary from 7 to 10 mm. in 
length. 
The examples of this species vary considerably in size and in details of outer 
form, but in a large series of specimens numerous transitional forms are present, 
connecting the extreme variations, thus showing the untenability of the species 
which by different authors have been founded on these individual differences. 
The specimens from British strata are so altered by fossilization, that it would 
be impossible from them alone to understand the original structures. Even their 
outer forms have been considerably compressed and distorted so as to appear as 
merely flattened plates or thin discs, or even as surface impressions only. The 
summit aperture is usually concealed, and the base of the Sponge is scarcely dis- 
tinguishable from the top. The spicular structures are now only represented by 
negative casts in the rock-matrix. Generally the surface of these specimens is 
