PROTOZOA 



93 



Orbitoides is very abundant in the lower Tertiary. In the 

 Gulf States of North America it makes up in places the entire 

 rock, just as Nummulites does in the Med- 

 iterranean region of Eurasia and Africa. 

 The Vicksburg or Orbitoides limestone of 

 Oligocene age attains a considerable thick- 

 ness in Alabama. ' '^ 



1. Sketch (a) surface view, (b) longitu- 

 dinal section, (c) transverse section. 



2. How does this differ from Nummulites? 



Sub-class 2, Actinopoda 



/no4.-!ir,Mriiii..ti 



l.c^- 



Usually floating forms with ray-like, un- 

 branched, rarely changeable pseudopodia 

 provided with an axial thread (w^hence the 

 name from Greek aktis (aktin-) sl ray, + pous 

 (pod) foot). This is subdivided into the 

 orders : 



a. Heliozoa, commonly called the " sun- 

 animalcules " because of the fine ray-like 

 pseudopodia (whence the name from Greek 

 helios, sun,+ zoon, animal). These do not 

 possess the chitinous central capsule char- 

 acteristic of the Radiolaria. 



Fig. 34. — A protozoan 

 calcareous skeleton, 

 Orbitoides, abounding 

 in the Oligocene sea 

 at St. Stephens, Ala- 

 bama. A , surface view 

 (X I). B, vertical sec- 

 tion along line a. 

 C, section of another 

 specimen much en- 

 larged, m.ch., median 

 chambers ; this row 

 is much larger than 

 the ones (l.ch.) upon 

 either side ; l.ch., lat- 

 eral chambers. 



Fig. 35. — Siliceous skeletons of 

 Radiolaria, from the Miocene of 

 Maryland. A, Hexalonche micro- 

 sphcera Vinassa ( X 130). B, 

 Cenosphara porosissima Vinassa 

 (X160). (From Martin.) 



They 



are almost confined to fresh water. 

 Unknown in the fossil state. Ex- 

 ample, Actinophrys. 



h. Radiolaria (Fig. 35). The 

 body is composed of a central mass 

 of protoplasm inclosed in a perfo- 

 rated, chitinous membrane, the 

 central capsule, and is continuous 

 through this with a layer of pro- 

 toplasm outside the membrane ; 

 this layer gives off ray-like pseu- 



