34 PORIFERA. 



four or five series of smaller ones. The spicules were 

 either free or probably partly fused together. Menevian 

 Beds and Lingula Flags. 



Ventriculites. Simple, form variable, but usually 

 cup-shaped, funnel-shaped, or cylindrical. Central cavity 

 large and deep. Walls folded so as to form a series of 

 vertical grooves and ridges. Canal-system well developed, 

 the radial canals are large and start from the surface of 

 the central cavity, but end before reaching the outer 

 surface, others start from the outer surface and end before 

 reaching the central cavity. Spicules six-rayed and fused 

 with one another so as to form a mesh-work. The node 

 where the axes of the spicule cross is hollow, having the 

 form of a negative octahedron, the central part of each 

 face of which is absent; the axial canals cross in the 

 centre of the space. The sponge was provided with a 

 root consisting of siliceous fibres. Upper Chalk. 



ORDER 7. OCTACTINELLID^. Spicules siliceous (fig. 

 6, g), consisting of eight rays, six of which are in one 

 plane diverging at equal angles, 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 Astrceospongia found in the Silurian and 

 Devonian. 



ORDER 8. HETERACTINELLHXE. Spicules siliceous 

 and unusually large (fig.. 6, h), the number of rays varying 

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

 there is a dermal layer in which the spicules are inter- 

 woven and more or less fused. The only genera are 

 Tholiasterella and Asteractinella found in the Carbon- 

 iferous rocks of Ayrshire. 



ORDER 9. CALCISPONGLE. Spicules composed of car- 



