FOSSIL WORMS. 169 



ness. Indeed, any geological student who has visited 

 the seashore of St. Bee's, Cumberland, at low water, 

 will have noticed extensive beds formed solely of the 

 cemented sand-tubes of modern species of Sabellaria. 

 Sabellaria and Terebella are very common tubed 

 worms in British seas, both of them constructing 

 sand-cemented tubes. The latter is always abundant 

 where there is a hard, clayey sea-bottom. It is rarely 



Fig. 138. Nereis (recent). 



that we get these worm-tubes fossilized, as they tend 

 to fall into their component grains of sand when the 

 worms die. 



With the hard, calcareous tubes of such species 

 of sea-worms as Serpula and its kind, we have no 

 difficulty. This form of Annelid has had, perhaps, a 

 more stereotyped or stable form of existence than 

 any other creature in the world. There is no osten- 



