FOSSIL SPONGES, ETC. 25 



radiatus being perhaps the commonest. This species 

 frequently forms the nucleus of flints, the flinty 

 matter just covering it, and thus assuming its shape. 

 The general shape of these Ventriculites is that of an 

 old-fashioned wine-glass. The stems and bases of 

 such an imaginaiy wine-glass (when thoroughly silici- 

 fied) are the well-known Choanites of the " Brighton 

 pebbles." A nearly allied and very elegantly marked 

 genus of sponges is Coscinipora. If we try to imagine 

 a Ventriculite flattened down into the similitude of a 

 finger-basin rather than a wine-glass, we shall form 

 a good idea of a much larger, nearly related, and 

 not uncommon hexactinellid sponge, called Cepha- 

 Utes. Other pretty fossil sponges belonging to this 

 division are Camerospongia^ Cceolptychium^ and Callo- 

 dictyon. 



The lithistid sponges are very abundant in some 

 parts of the Lower Cretaceous rocks. The principal 

 genera are Siphonia, Polypothecia^ Scyphia, etc. The 

 former genus is perhaps the commonest, and there 

 are several species classified as belonging to it. Most 

 young geologists are acquainted with the pear-shaped, 

 stony heads of a species called SipJionia pyriformis. 

 Very frequently they break off the stalks, and we find 

 head, stalk, and roots separately. In the " fire- 

 stone" of St. Catherine's Hill, above the road from 

 Niton, Isle of Wight, we may obtain any quantity of 

 Siphoftia Websteri ; near Ventnor, S. pyriforme is 

 abundant. In the Upper Greensand at Farnham, in 



