170 OUR COMMON BRITISH FOSSILS. 



sible difference between many Silurian-tubed worms 

 and those now in existence. The pretty little 

 Spirorbis found in the Upper Silurian rocks has had 

 a continuous and unchanged existence through every 

 geological period until now. We find it attached to 

 fossil shells and corals in the Silurian and Devonian 

 limestones. Fourteen species occur in the Carboni- 

 ferous rocks, some of them found adhering to the 

 trunks of Sigillaria and almost encrusting them, 

 just as we find them adhering to the larger seaweeds 

 along our shores. Under the names of Serpidites, 

 Cornulites, Tentaculites^ Conchiolites, etc., the sea of 

 every geological period has abounded in tubed worms. 

 The Wenlock limestone literally swarms with these 

 tapering elegant tubes, ringed like the tentacles of 

 insects, and hence called Tentacidites, This is a type 

 of what is called 2. free worm-tube, i.e. one that is not 

 attached to shells or rocks, like the modern Serptila. 

 By some geologists it is still regarded as a pteropod 

 mollusc allied to Hyale. Cornulites is another genus 

 nearly related to it, and both are characteristic of the 

 Silurian formation. The chief species in the latter 

 rocks is Cornulites serpulariiis. It is frequently found 

 as much as three to four inches in length, ringed, 

 and gradually tapering to a point. Casts of this 

 species of worm-tube frequently occur, and the young 

 student might be easily misled by them into thinking 

 it was a different fossil to the Cornulites found with 

 its external shape ; for this internal cast is in a series 



