PRE-CAMBRIAN LIFE 



63 



sandstones, would seem to have afforded favourable 

 habitats, and warrant the expectation that species 

 may yet be found. 



The case was different with the little group of 

 the Lamp-shells, or Brachiopods. These creatures, 

 somewhat resembling the ordinary bivalves in their 

 shelly coverings, were very dissimilar in their in- 

 ternal structure, and once settled on the bottom they 

 were attached for life, not having even the limited 

 means of locomotion possessed by the Sea-snails 

 and common bivalves. They collected their food 

 wholly by means of currents of water produced by 

 cilia, or movable threads, on arms or processes 

 within their shells. In this they resembled the 

 young or embryo stages of some of the more ordin- 

 ary Mollusks, though they are so remote from these 

 in their adult condition that they have usually been 

 placed in a distinct class, and some naturalists have 

 thought it best to separate them from the Mollusks 

 altogether. Their history is peculiar. Coming into 

 existence at a very early date, they became ver)' 

 abundant in early Palaeozoic times, then gradually 

 gave place to the ordinary bivalves, and in the 

 modern seas are represented by very few species. 

 Yet while in the middle period of their history they 



