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 ScoliorhapMs, 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 1 from the Lower Lias of Schafberg in the Tyrol, by the 

 writer 2 in the Lower and Upper Greensands in this country, and by Rutot 8 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 Ivais. Akad. der Wiss. Wien,' 1SS2. 



2 « Phil. Trans.,' Pt. ii, 1S85. 



3 ' Ann. de la Soc. Malacol. de Belgique,' Tome ix, 1874. 



