PALAEONTOLOGY 5 



PEOTOZOA. 



Class I.— AMORPHOZOA. 



Fussil sponges take an important place among the organic 

 remains of the former world, not only on account of their 

 great variety of form and structure, but still more because of 

 the extraordinary abundance of individuals in certain strata. 

 In England they specially characterize the chalk formation, — 

 extensive beds of silicified sponges occur in the upper green- 

 sand, and in some beds of the oolite and carboniferous lime- 

 stone. In Germany a member of the Oxford oolite is called 

 the " spongitenkalk," from its numerous fossils of the present 

 class. 



Existing sponges are divided into horny, flinty, and limy, 

 or "ceratose," "silicious," and "calcareous," according to the 

 substance of their hard sustaining parts, which parts are 

 commonly in the shape of fine needles, or spicula, of very 

 varied forms, but in many species of sufficient constancy to 

 characterize such species. The soft organic substance called 

 "sarcode" appears to be structureless, and is diffluent; it is 

 uncontractile and impassive, but consists of an aggregate of 

 more or less radiated corpuscles, in some of which the trace 

 of a nucleus may be discerned. The larger orifices on the 

 surface of a sponge are termed "oscula," and are those out of 

 which the currents of water flow : these enter by more nume- 

 rous and minute "pores." 



The calcareous sponges abound in the oolitic and creta- 

 ceous strata, attaining their maximum of development in the 

 chalk; they are now almost extinct, or are represented by 

 other families with calcareous spicula. The horny sponges 

 appear to be more abundant now than in the ancient seas, but 

 their remains are only recognisable in those instances where 

 they were charged with silicious spicula. 



