MODE OF PRESERVATION. 63 
Modifications in Fossil Sponges resulting from their Spicular Structures.— 
Hitherto we have considered the changes produced in fossil Sponges by the 
chemical alteration of their skeletal constituents, but there are other modifications 
arising from the way in which their skeletal elements are built up, which have an 
important bearing on the preservation of the Sponge in the fossil state. Sponges 
in which the skeletal elements are free from each other, and merely held in position 
by the soft, fleshy structures of the living organism, cannot, except under 
very favorable circumstances, be preserved entire in the fossil state, but their 
skeletal spicules, on the death of the animal, and the decay of the soft tissues, 
become detached and indiscriminately mingled together in the ooze of the sea- 
bottom. On the other hand, Sponges in which the skeletal spicules are closely and 
intimately connected, or organically united together, retain their entire forms, 
other conditions being favorable, in the fossil state. 
Thus the component spicules in monactinellid and tetractinellid Sponges are 
either only loosely embedded in the soft tissues, or they form anastomosing fibres 
enclosed in perishable spongin, and as a result entire Sponges belonging to these 
groups are very rare in the fossil state. As exceptional examples may be mentioned 
the Carboniferous genus Haplistion, Young, and from the Cretaceous strata of 
Germany Opetionella, Zitt., and Scoliorhaphis, Zitt., whilst some forms of Ophira- 
phidites, and Pachastrella occur in the Upper Chalk in this country. In these 
cases the free spicules are very thickly disposed amongst each other in the body of 
the Sponge, and the Sponges appear to have been undisturbed whilst they were 
gradually covered with sediment. In some instances the spicules have been subse- 
quently cemented together by a secondary deposition of silica. In contrast to the 
rare occurrence of entire Sponges of these groups as fossil, is the abundance of 
their detached spicules, which, intermingled together, form extensive beds, like those 
described by Dunikowski' from the Lower Lias of Schafberg in the Tyrol, by the 
writer’ in the Lower and Upper Greensands in this country, and by Rutot* in the 
Eocene of Belgium. They are also similarly abundant in the Upper Chalk. These 
detached spicules evidently belong to various genera and species of Sponges, but 
their entire forms have been wholly destroyed. 
In lithistid Sponges, in which the component spicules are more or less 
intimately united together by the interlacing of their branching extremities, the 
entire form of the Sponge is usually preserved, though in those belonging to the 
Megamorina family, in which the union of the spicules is of a simpler character, the 
Sponges have, to a great extent, been disintegrated, and their component spicules 
occur detached in great numbers in the Lower and Upper Greensands and in the 
1 ¢Denkschr. der Kais. Akad. der Wiss. Wien, 1882. 
2 ¢ Phil, Trans.,’ Pt. ii, 1885. 
3 “Ann, de la Soc. Malacol. de Belgique,’ Tome ix, 1874. 
