PORIFERA. HEXACTINELLIDA 39 



and also occurs in the Ordovician, Silurian, and Car- 

 boniferous. In the Silurian the genera Dictyophyton and 

 Phonnosella are present. There are none in the Permian 

 and Trias, but they become abundant in the Jurassic, 

 especially in the upper part, and also in the Cretaceous ; 

 they are rare in the Tertiary. 



Protospongia. (fig. 10.) Form unknown but probably cup- 

 shaped. Spicules cruciform owing to the reduction of one axis, and 

 arranged in a quadrate manner, the larger forming a framework, 

 which contains the smaller spicules of two or three sizes, arranged in 

 the same regular way, so that the larger squares enclose four or five 

 series of smaller ones. The spicules were either free or probably 



■A.:' 



Vv 





^::^v%i 



Fig. 10. Protospongia fenestrata, Menevian Beds, St David's. x 3|. 

 (After Hinde.) The original is in the British Museum. Owing to 

 the cleavage of the rock the angles of the spicules are distorted. 



partly fused together. Menevian Beds and Lingula Flags. Ex. 

 P. fenestrata. 



Craticularia. Cup-shaped or cylindrical ; simple or branch- 

 ing. On both the inner and outer surfaces of the wall are circular 

 or oval canal-openings, which are arranged in vertical and transverse 

 rows crossing each other at right angles. Canals straight, terminating 

 blindly Inferior Oolite to Upper Chalk (perhaps also Miocene). 

 Ex. C. Jittoni, Chalk. 



Ventriculites. Simple, form variable, but usually cup- 

 shaped, funnel-shaped, or cylindrical. Central cavity large and 



