ANNELIDA. 



315 



surface. Species of this genus have been described from the 

 Upper Silurian, Devonian, and Carboniferous l formations. 

 In the Trias some species occur, and many forms are known 

 from the Oolitic and Cretaceous formations, whilst they are 

 equally numerous in the Tertiary series. 



Very nearly allied to Serpula, and probably hardly, if at 

 all distinguishable in the fossil condition, are the genera 

 Filograna and Vermilia. Taking these three genera together, 

 the occurrence of Tubicolar 

 Annelides of this particular 

 .type in past time must be 

 said to be fairly frequent, 

 though remains of this na- 

 ture are naturally more nu- 

 merous in Mesozoic and 

 Kainozoic strata than in de- 

 posits of Palaeozoic age. 



Of the other fossils re- 

 ferred to the Tubicolar An- 

 nelides, the only one which 

 needs notice is the genus 

 Ditrupa. The tube in this 

 genus is unattached, open 

 at both extremities, and very 

 closely resembling the shell 

 of a Dentalium. This genus 

 does not seem to have come 

 into existence till the close 

 of the Cretaceous period; but it is found in great abun- 

 dance in the London Clay (Eocene) and the Crag (Pliocene). 



ORDER ERRANTIA. The Errant Annelides are character- 

 ised by the fact that the ~body is covered with lateral tubercles 

 carrying tufts of bristles. The animal (fig. 188) leads a free 

 life, and is not confined to a tube. The gills are placed along 

 the back or sides of the body ; hence the name of " Dorsibran- 

 chiate Annelides" often applied to this order. 



1 The Serpula parallela of M'Coy, of the Carboniferous rocks, is now known 

 to be really the siliceous "rope" of a Sponge (Hyalonema). 



Fig. 187. Serpula flagellum. Oxford Clay 

 (Jurassic). 



