THE GEOLOGIC FUNCTIONS OF LIFE. 661 



water. Foraminifera live in shallow water as well as in the open sea, 

 and in this case they sometimes creep on the bottom or are attached to 

 algse, but their deposits in shallow water are usually much obscured by 

 other kinds of deposition and by destructive action. Some of the for- 

 aminiferal shells are divided into chambers and assume various spiral 

 forms, of which the Nummulites, named from their resemblance to 

 coins, are notable examples. These formed an important part of the 

 hummulitic limestone of the Eocene period. 



The radiolarian ooze is characterized by the silicious tests of various 

 members of the silica-bearing order, Radiolaria. The '^Barbadoes 

 earth" and '^Tripoli" are notable deposits of fossil radiolarians. 



The contribution of the Coelenterata. — The Coelenterata embrace the 

 sponges, the coral polyps (Anthozoa), and the hydroids and medusae 

 (Hydrozoa). The contribution of coral polyps to the formation of lime- 

 stone is most important, and is too familiar to require elaboration here. 

 The corals range throughout nearly the whole fossiliferous series, and 

 their development will be followed and illustrated in the historical 

 chapters. 



The sponges are widely represented by their spicules, and not uncom- 

 monly their aggregate form is preserved even in very ancient strata. 

 Their contribution is largely silicious, but is partly calcareous. The 

 hydroids and medusae have left little trace of themselves in the rocks, 

 although impressions supposed to represent medusae are found in strata 

 as early as the Cambrian. Certain coral-like forms, as the Millepores, 

 Tubularia, and Stromatopora, are classed as Hydrozoa. The grapto- 

 lites, delicate leaf-like floating forms, very serviceable in marking exact 

 horizons on different continents because of their free distribution, are 

 also classed here. 



The contribution of the Echinodermata. — Under the echinoderms are 

 grouped the crinoids (sea-lilies), cystoids, blastoids, ophiuroids (brittle 

 stars), asteroids (starfishes), echinoids (sea-urchins), and holothuroids 

 (sea-cucumbers). This is one of the marked groups of ancient as well 

 as modern life, and its beautiful fossils grace every period in which life 

 relics are w^ell preserved. The cystoids and crinoids, and later the 

 blastoids, were prominent in the Paleozoic ages, while the remaining 

 forms were more conspicuous later, though earh^ introduced. All 

 divisions, except the holothuroids, whose softness prevented, have left 

 a good record, as fossil records go. Their relics are chiefly calcareous, 



