MATERIALS 49 



solvable, impressions of collapsed jelly-fish or partly 

 decayed Holothurians are none the less distinctive, 

 whether the period of their production was Holocene, 

 Jurassic or Cambrian. Although such traces rarely 

 provide opportunity for appreciation of the anatomical 

 qualities of the animals that made them, they at least 

 prove the existence of various types (often incapable of 

 direct fossilization) in remote periods. It is interesting 

 to find that " worm-tracks " occur well down in the Pre- 

 Cambrian of the Longmynd and elsewhere, in slates 

 that contain no other recognizable fossils. These rough 

 markings thus carry back indications of life (and that 

 of a fairly high order) far beyond the limits at which 

 ordinary palaeontological evidence fails. Fine Cambrian 

 silt in British Columbia has yielded wonderfully clear 

 impressions of Medusae and Holothurians, while the 

 Upper Jurassic Lithographic stone of Solenhofen includes 

 similar relics. 



Closely akin to impressions of past organisms are the 

 tubular aggregates of fish-scales found in the Chalk, and 

 ascribed to the activities of marine Annelids. These 

 protective tubes recall the familiar " caddis-cases " of 

 modern rivers. They afford some evidence as to the 

 fishes of the period, and suggest the existence of worms ; 

 but they are hardly satisfactory as fossils of either group 

 of organisms. 



