14 



NORTH AMERICAN INDEX FOSSILS. 



SYNOPSIS OF IMPORTANT GENERA AND SPECIES. 



I. Astylospongia Roemer. 

 Free, spherical, with a shallow depression on top. Base evenly 

 rounded. Surface furrowed by large canals and pitted with pores, 



Fig. 22. Astylospongia prcemorsa, with enlarged section (Roemer). 



which terminate many fine radial canals. Spicules four-rayed and 

 branching, with their nodes enlarged into knots. Siluric. 



1. A. praemorsa Goldfuss (Fig. 22). Siluric. 

 Summit shallow and pierced with many large holes. From the 



edge of this depression radiate shallow furrows irregularly over the 

 sides. 



Niagara of Indiana, Tennessee, etc. 



2. A. inciso-lobata Roemer. Siluric. 

 Differs from A. prcemorsa in the absence of large pores above and 



in that the furrows number but six to eight and are deeply incised, 

 thus dividing the surface into large, rounded lobes. 

 Niagara of Tennessee. 



II. Hindia Duncan. 

 Free, spheroidal. Spicules form a series of bifurcating, straight 

 canals radiating from the center and opening at the surface. 

 Siluric. 



3. H. fibrosa Goldfuss. Ordovicic-Siluric. 

 Ball-shaped, about one inch in diameter. Surface covered with 



very small, irregular, polygonal openings. 



Reported from the middle and upper Ordovicic, the Siluric and 

 even the Helderbergian. 



III. Dictyospongia Hall and Clarke. 

 Subcylindrical, very gradually expanding. Smooth, with no 

 ornamentation except the reticulated meshwork of spicular threads. 



