12 OUR COMMON BRITISH FOSSILS. 



the former appear to be more numerous and wide- 

 spread. Dr. Hinde refers to instances where the 

 spicules of Tetractinellid, or four-rayed sponges, are 

 numerous enough to form thin strata. 



The Hexactinellid sponges are distinguished by 

 having six-rayed spicules, so arranged that the rays are 

 usually at right angles to each other. These spicules 

 are sometimes united, sometimes free, and they 

 frequently form a trelliswork after the appearance 

 of " Venus' flower-basket." These are all inhabitants 

 of the deepest parts of the sea, and as our white 

 chalk is one of the geological formations deposited 

 perhaps in deeper water than any other, the number of 

 species of fossil hexactinellid sponges found fossilized 

 in its strata is very great. The Ventriculidce (formerly 

 thought to be zoophytes allied to the Alcyonia, or 

 " dead men's fingers ") are perhaps the most 

 numerous of all the chalk sponges, and even the 

 external shapes of many of them approach very near 

 to that of " Venus' flower-basket," and still more 

 to the recent sponge called Holtenia. 



The Lithistidce are sponges whose spicules are 

 also composed of silica, but they are arranged in fotirs, 

 and the rays are not arranged at right angles. The 

 extremities of the spicules are usually so divided 

 or notched that those of each other can interlock. 

 This interlocking builds up a loose but continuous 

 framework. One of the commonest of the fossil 

 sponges in the Lower Chalk is Siphonia, which 



