18 NORTH AMERICAN INDEX FOSSILS. 



tively large, eight-rayed spicules, six of which are disposed in the 

 same plane while the two projected at right angles to these are 

 reduced to button-like prominence. Siluric. 



8. A. meniscus Roemer. (Fig. 27.) Siluric. 



Fig. 27. Astrceospongia meniscus with enlargement of spicule c (Roemer). 



Vertical section crescent- (meniscus) shaped. Star-shaped spic- 

 ules very conspicuous, especially on the concave surface. 

 Upper Siluric of Tennessee. 



VIII. Receptaculites Defrance. 



Globular to platter-shaped bodies, containing a central cavity 

 whose calcareous wall is composed of five-rayed spicules. Ord. to 

 Dev. 



9. R. oweni Hall. (Fig. 28.) Ordovicic. 



Broad, varying in diameter from a few inches to two feet. Con- 

 sisting of a flat, circular disc with a small, funnel-shaped central 

 depression above, corresponding to the narrow projecting base of 

 attachment on the under side. Cell rows curving strongly as they 

 radiate from the center. Cell apertures quadrangular at surface, 

 becoming circular and decreasing in diameter towards the center. 



