GENERAL CHARACTERS. AT 
appendages are commonly present in fossil lithistids and hexactinellids, but in 
fossil calcisponges the attachment more generally occurs by means of a basal ex- 
pansion. As more distinctive anchoring appendages may be mentioned the ropes 
or bundles of long spicular rods proceeding from the base of the Sponge, and 
penetrating into the bottom ooze, as in the hexactinellid genus Hyalostelia (PI. 
VI, figs. 2, 3 d). 
Though the majority of fossil Sponges appear to have been furnished with 
means of attachment, others were undoubtedly free in their mature condition, and 
seem to have merely rested on the sea-floor without being in any way definitely 
fixed. This is shown by the entire, evenly rounded, and sometimes concave form 
of the basal portion of the skeleton. Prof. Ferd. Roemer has noted that this free 
condition is a well-marked peculiarity in most of the Sponges from Paleozoic strata. 
It is well shown in Astylospongia (PI. I, fig. 5), and Aulocopium ; in the different 
genera of the family Receptaculitide (Pl. HU, figs. 1, 2), im Amphispongia oblonga, 
Salt., (Pl. II, fig. 3) in Phormosella ovata, H. (Pl. UI, fig. 2), and in Dictyophyton 
Danbyi, M‘Coy sp. (Pl. I, fig. 4). It also occurs in some Cretaceous Sponges, as, 
for example, in the hexactinellid genus Stawronema, and in the hithistid, Plinthosella 
squamosa, Zitt. 
Asa general rule, fossil Sponges with thin walls are hexactinellids, but many 
forms in which the walls are apparently thick also belong to this same group. In 
this latter case, however, it will usually be found that the apparent thickness 
is really due to numerous closely arranged foldings of a simple thin wall. In 
lithistid Sponges, on the other hand, as already remarked by Zittel, the walls are 
thicker and of a more compact, firm, and stony character, so that they bear more 
general resemblance to the coenenchymal tissue of Corals, than to that of normal 
Sponges. As regards the compact structures of the skeleton, fossil calcisponges 
resemble lithistids, but in general they can be readily distinguished from Sponges 
of this latter group, leaving on one side the different nature of their spicular 
skeletons, by their smaller dimensions and the continuous reticulation of the fibres 
on their outer surfaces. These distinguishing features of the outer form and 
general appearance in the different groups of fossil Sponges are frequently largely 
masked by their condition of preservation, but they will sometimes afford a clue to 
the real character of the Sponge, when its spicular structure has been destroyed. 
In many fossil calcisponges the exterior or under surface is partially or entirely 
covered by a smooth, even, or corrugated membrane, resembling in general aspect 
the epitheca of Corals, whilst the upper surface is furrowed and uneven, and 
destitute of this dermal layer. A similar structure is occasionally present in 
lithistid and hexactinellid Sponges. 
