THE ORDOVICIAN PERIOD. 



361 



greater part they belonged to the simpler horn-shaped type (Fig. 166, 

 a, b, and c). Compound corals, formed by colonies of polyps living 

 together, were present (Fig. 166, d and e) and rarely formed reefs. 

 The hydrozoan branch of the ccelenterates was represented by Stroma- 

 topora, which deposited thin films of calcium carbonate upon one 

 another concentrically, giving rise to laminated masses somewhat 

 resembling concretions and often distinguished from them with diffi- 

 culty. 



Much the most important development of the Coelenterates was 

 the rise of the graptolites, whose important function in correlation has 



Fig. 166. — Ordovician Corals, a-c, Streptelasma cornisrulum Hall, exterior view with 

 longitudinal and transverse sections, showing internal characters, d, e, Columnaria 

 alveolata Goldf. While both simple and compound corals were present in the 

 Ordovician faunas, they rarely formed great reef-like masses, as in later periods. 



already been discussed, and which will be further considered presently. 

 Some of the typical forms are illustrated in Fig. 167. 



Other forms. — Sponges were present and sometimes attained nota- 

 ble sizes, but they played only a small part in the fauna (Fig. 168). 

 Annelids were represented, but less abundantly than in the Cambrian, 

 perhaps because the prevalently muddy and calcareous sea-bottom of 

 the Ordovician was less congenial to them than the Cambrian sands. 

 They are represented not only by burrows, but by teeth (Fig. 169). 

 Protozoans were probably present, but their minute and fragile shells 

 can be recognized only with some uncertainty. 



Implied life. — If here, as in the discussion of the Cambrian life, we 

 inquire what forms other than those fossilized are necessary to round 

 out a rational assemblage of life, a briefer answer may be given, for the 

 Ordovician fauna was a nearer approach to a theoretically complete 

 assemblage. Avast supply of unrecorded vegetation must be postulated 

 as a food-supply, as before, and to provide a complete chain of food- 

 supplies for each one of the whole series of organisms that preyed upon 



