THE EARLIER FORMS OF LIFE. 



263 



this calcareous mixture becomes solidified it is the chalk which 

 abounds in the Cretaceous formation of Europe, and makes up the 

 nummulitic and orbitoidal limestones. 



Fig. 2 is a close copy of a small slab of Eozoon, showing what are 

 called the laminated, acervuline, and fragmental portions. The 

 diagonal white line represents the course of a vein of calcite. The 

 dark lines and marks correspond to the sarcode or animal matter of 

 the animal, now consisting of serpentine. Calling the base of the 



Fig. 2. Natuke-Print op Eozoon. (Dawson.) 



*figure the ocean-floor, there may be said to grow upon it the gelatinous 

 sarcode or dark mass. Upon it grew first the delicate calcareous shell, 

 penetrated by the numerous minute orifices or tubuli, larger pores, 

 and occasionally supports of perpendicular plates. Added to this is 

 the supplemental skeleton without the minute tubuli, but traversed 

 by branching canals. This whole skeleton is represented by the 

 white mass next the dark base, consisting of calcite. These two lay- 

 ers or laminae constitute the. essential part of the structure, and all the 

 numerous layers above are but repetitions of them. Each lamina may 

 cover several inches square of surface at the bottom of the ocean, and 

 perhaps diminish in size as the organism grew upward. In the sketch 

 the layers are seen to grow thinner toward the top, as if the vital 

 energy became exhausted by the demands made upon it, and the sup- 

 plemental skeleton first disappears. Finally, we have a mass of 

 rounded chambers irregularly piled up near the top, constituting the 

 " acervuline " structure. We may suppose the growth arrested at 

 this stage, and the sending forth of reproductive germs to found new 

 colonies in the adjacent ground. 



