LIFE IN THE EARLY CAMBRIAN 29 



they also abound in those greater depths to which 

 voyagers have only recently had access. Sea-worms 

 seem thus to be able to live in all depths, as well 

 as in all climates ; and in accordance with this 

 they abound in the oldest rocks, which are often 

 riddled with the holes caused by their burrowing, or 

 abundantly marked on the surfaces of the beds with 

 their trails. 



The great province of the Mollusca, in which, 

 for our present purpose, we may include some 

 aberrant and rudimentary Molluscoids, is now best 

 known to us by its medium types, the univalve and 

 bivalve Shell-fishes ; the higher group of the Cuttle- 

 fishes and Nautili, though not uncommon, being 

 much less numerous, and one at least of the lower 

 groups, the Lamp-shells or Brachiopods, being repre- 

 sented in the modern world by but few forms. The 

 extension of the Mollusks backwards into the Cam- 

 brian is remarkable as being on the whole meagre 

 in comparison with that of the Crustaceans, and as 

 presenting only in small numbers the types most 

 common in later times. One or two shells, and 

 perhaps some tracks, represent the highest group : 

 some forms resembling the floating species of Sea- 

 snails, and a very few ordinary bivalves represent 



