LOWER PALAEOZOIC FAUNAS 127 



(B) PROTOZOA 



The relative simplicity of the Protozoa would suggest 

 that they were abundant, if not predominant, members 

 of early faunas. Although such a condition may have 

 been realized, the fact remains that relics of the two 

 orders capable of fossilization are less abundant in the 

 Lower Palaeozoic than at later periods. Radiolaria 

 occur sparingly in all three stages (and, indeed, in the 

 pre-Cambrian), but Foraminifera are unknown in the 

 Cambrian, barely recorded from the Ordovician, and 

 exceedingly rare in the Silurian. Two explanations 

 of this anomaly are available. The indurated quality 

 of most Lower Palaeozoic rocks makes separation of 

 minute fossils difficult; further, it has resulted from 

 incipient metamorphic changes that would normally 

 obscure small objects by molecular readjustment. The 

 persistence of siliceous Radiolaria suggests the opera- 

 tion of this process, since their substance would offer 

 resistance to alteration. Secondly, the retarded nature 

 of Protozoan evolution may have found expression 

 in delay in the secretion of hard structures. The 

 almost complete absence of Foraminifera from Ordo- 

 vician deposits seems explicable only on the assumption 

 that few members of the order possessed durable tests 

 at that time. Radiolaria are locally present in rock- 

 forming quantities in the Ordovician of Scotland. 

 Additional support for this belief is found in the nature 

 of the least rare Silurian Foraminifera. Saccammina 

 and Placopsilina are both " Agglutinantia," building 

 tests in which adherent foreign particles are bound 

 together by an insignificant amount of secreted matter. 

 The few vitro-calcareous genera of the period are usually 

 found as glauconitic moulds; while this character is 

 shown even by Holocene types, it is suggestive of 



