44 



PORIFERA. DEMOSPONGI.E 



Hallirhoa. Like Siphonia but with the sides divided into 

 lobes. Upper Greensand. Ex. H. costata. 



Order 6. Octactinellida. The spicules (fig. 9, g), 

 consist of eight rays, six of which are in one plane 

 diverging at equal angles, while the other two are at right 

 angles to this plane, forming a vertical axis. Frequently, 

 however, the vertical axis is only slightly developed or 

 altogether absent. The spicules are not united. The 

 only genus is Astrcvospongia, found in the Silurian and 

 Devonian. 



i— 



B 



Fig. 11. Siphonia tulipa. Upper Greensand, Warminster. A, vertical 

 section. B, horizontal section, e, excurrent canals ; i, incurrent 



canals, x £, 



Order 7. Heteractinellida. The spicules are un- 

 usually large (fig. 9, h), the number of rays varying from 

 six to thirty. The body spicules are not fused, but there 

 is a surface layer in which the spicules are interwoven and 

 more or less fused. The only genera are Tholiasterella 

 and Aster actinella, found in the Carboniferous rocks of 

 Ayrshire. 



