G PALAEONTOLOGY. 



Desmidiw, Gregarinw, and most of the Polygastria of Ehren- 

 berg, or infusorial animalcules of older authors. 



ACEITA or Protozoa. 

 Class I. — Amorphozoa.* 



Fossil sponges take an important place among the organic 

 remains of the former world ; less on account of their great 

 variety of form and structure, than because of the extraordi- 

 nary abundance of individuals in certain strata. In England 

 they specially characterize the chalk formation : extensive 

 beds of silicified sponges occur in the upper greensancl, 

 and in some of the oolitic and carboniferous limestones. 

 In Germany a member of the upper 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 (t 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 



* Gr. a, without ; morplie, form ; zoe, life. 



