PORIFERA. 33 



the centre of the sponge to the surface. The excurrent 

 canals are larger, and are arranged parallel to the surface 

 of the sponge, extending from the base to the summit, 

 where they open into the large central cavity by means 

 of a series of parallel ostia. The skeletal-spicules possess 

 four rays with bifurcated and expanded extremities, by 

 means of which they interlock. Upper Greensand to 

 Upper Chalk. 



ORDER 6. HEXACTINELLID.E. The spicules (fig. 6,/) 

 are siliceous and consist of six rays (three axes) at right 

 angles to one another. Each ray is traversed by an 

 axial canal, and these unite at the point of junction of the 

 six rays. In some cases one ray is longer or shorter than 

 the others, or almost absent. The spicules are united 

 so as to form a framework of skeleton cubes, either by 

 being fused, or simply held in position by the soft parts. 

 Flesh-spicules are abundant, but rare as fossils. The 

 canal-system is usually simple. The earliest form is 

 Protospongia from the Menevian Beds of St Davids. In 

 the Ordovician and Silurian we get the genera Dictyophyton 

 and Phormosella ; in the Carboniferous Hyalostelia. 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. 

 The sponge -character of the genera Ischadites, Receptacu- 

 lites, and Sphcerospongia, from the Silurian and Devonian 

 is now disputed. 



Protospongia. Form unknown but probably cup- 

 shaped. Spicules cruciform, owing to the reduction of 

 one axis, and arranged in a quadrate manner, the larger 

 ones forming a framework, in the squares of which are 

 smaller spicules of two or three sizes, arranged in the 

 same regular way, so that the larger squares enclose 

 w. P. 3 



