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 Hexactinellidze, 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, AmpHISPoNnGia oBLONGA, Salter. Plate III, figs. 3, 3 a—3 f. 
1861, ApHisponata oBLoNGA, Salter. Mem. Geol. Survey Great Britain, 
Sheet 82 Scotland, p. 135, t. 2, f. 38. 
1872. - — Murchison. Siluria, 4th edit., p. 509. 
1877. — — Zittel. Studien, Abtheil. 1, p. 45, Note. 
1879. _ — Nicholson. Manual of Paleontology, 2nd ed., 
vol. 1, p. 135, figs. 33.¢, d. 
1880. — — I’, Roemer, Lethea pal., p. 317. 
1883. a — Hinde. Cat. Foss. Sponges, p. 154, pl. xxxiii, 
figs. 12, 12 a—12 d. 
