AMPHISPONGIA. 131 



dovetail into each other and form a close, even spicular tissue. There are traces 

 of slender filiform spicules forming an outer layer to the upper portion of the 

 Sponge. The spicules are not organically united together. 



Mr. Salter regarded Amphispongia as a Calcisponge, and Dr. Bowerbank held 

 the same opinion. The spicules of the upper portion of the Sponge were 

 described as three-rayed forms similar to those of recent Calcisponges, whilst those 

 of the basal portion were stated to be bundles of simple spicules. The fact that in 

 all cases the spicular structures are now only represented by moulds or casts, 

 which are mostly empty or occasionally refilled with a loose powdery iron-rust, 

 not only gave rise to the mistake as to the form of the spicules but also confirmed 

 the idea that they must have been originally of carbonate of lime, since at the 

 time when Salter described this genus it was generally supposed that siliceous 

 fossils would not be liable to dissolution the same as structures of calcite. It is 

 clear, however, from the form of the spicules and from the presence of five rays 

 in many of them, that they are related to siliceous hexactinellids, and the existence 

 of undoubted siliceous Sponges, such as Plectoderma for example, in these same 

 beds, and for the most part in a similar condition of preservation, shows the 

 untenability of the theory that since the spicules are dissolved they must neces- 

 sarily have been originally calcareous. 



But whilst the character of the spicules of the upper portion of the Sponge 

 shows its relationship to the Hexactinellidge, the mode in which they are interwoven 

 together is altogether distinct from that of any other member of this group, and 

 the presence and the arrangement of the peculiar conical spicules in the basal portion 

 of the Sponge is similarly abnormal, so that Amphispongia stands quite apart from 

 other hexactinellids. 



The genus is only represented by a single species, which appears to be limited 

 to a definite stratum of decayed limestone of Upper-Ludlow age in the Pent- 

 land Hills. 



14. Amphispongia oblonga, Salter. Plate III, figs. 3, 3 a — 3/. 



1861. Amphispongia oblonga, Salter. Mem. G-eol. Survey Great Britain, 



Sheet 32 Scotland, p. 135, t. 2, £. 3. 

 1872. — — Murchison. Siluria, 4th edit., p. 509. 



1877. — — Zittel. Studien, Abtheil. 1, p. 45, Note. 



1879. — — Nicholson. Manual of Palaeontology, 2nd ed., 



vol. i, p. 135, figs. 33 c, d. 



1880. — — F. Roemer. Lethsea pal., p. 317. 



1883. — — Hinde. Cat. Foss. Sponges, p. 154, pi. xxxiii, 



figs. 12, 12 a— 12 d. 



