GENERAL CHARACTERS. 47 



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 Palaeozoic strata. 

 It is well shown in Astylospongia (PI. II, fig. 5), and Aulocopium ; in the different 

 genera of the family Receptaculitidce (PI. II, figs. 1, 2), in Amplrispongia oblonga, 

 Salt., (PI. Ill, fig. 3) in Phormosella ovata, H. (PL III, fig. 2), and in Dictyophyton 

 Danbyi, M'Coy sp. (PL II, fig. 4). It also occurs in some Cretaceous Sponges, as, 

 for example, in the hexactinellid genus Stauronema, and in the lithistid, PI intho sella 

 squamosa, Zitt. 



As a 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 ccenenchymal 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. 



