CGELENTERATA 



463 



beautiful structure may be made out with the microscope. In 

 some localities these sponges are heaped up in such masses that 

 they fill the strata, while other localities of the same horizon are 

 entirely free from them. 



Plate IX. American Jurassic Fossils 



1. Segment of stem of Pentacrinus asteriscus, 4/1. 2. Gryphaea nebrascensis. 

 3. Quenstedioceras cordiforme, 2/3. 4. Belemnites densus, 2/3. 5. Trigonia ameri- 

 cana. (After Meek and Hayden.) 



Coelenterata. — Corals abound, especially in the Upper Jurassic 

 of central Europe. The Anthozoan Corals all belong to the mod- 

 ern Hexacoralla, in decided contrast to the Tetracoralla of the 

 Palaeozoic seas. Isastrcea, Montlivaultia, and Thecosmilia are the 

 dominant genera. 



