ISCHADITES. 121 



1875. Ischadites Kcenigii, Gilmhel. Beitrage Abhandl. k. bayer. Akad. Wiss., 



Bd. xii, Abtheil. i, p. 43, pi. A, figs. 28, 

 29, 30. 



1878. — — Nicholson and Etheridge,jun. Silur. Foss. Girvan, 



p. 20. 



1878. — — Quenstedt. Petref. DeutscbL, vol. v, p. 592. 



1880. — Zittel. Handb. der Pal., vol. i, p. 728. 



1880. — — F. Roemer. Letha;a Pal., p. 291. 



1882. — — Rupert Jones. Cat. Foss. Foram. Brit. Mus., p. 2. 



1884. — — Hinde. Quart. Journ. Greol. Soc, vol. xl, p. 836, 



pi. xxxvi, figs. 1, la — 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 



