THE PRIMORDIAL, OR CAMBRIAN AGE. 43 



remarkable, in connection with our own very slender 

 acquaintance with the phenomenon, in relation either 

 to its efficient or final causes. 



Before leaving the Lingulae, I may mention that 

 Mr. Morse informs me that living specimens, when 

 detached from their moorings, can creep like worms, 

 leaving long furrows on the sand, and that they can 

 also construct sand-tubes wherein to shelter them- 

 selves. This shows that some of the abundant "worm 

 burrows " of the Primordial may have been the work 

 of these curious little shell-fishes, as well as, perhaps, 

 some of the markings which have been described 

 under the name of Eopliyton, and have been supposed, 

 I think incorrectly, to be remains of land plants. 



In addition to Lingula we may obtain, though 

 rarely, lamp- shells of another type, that of the Orthids, 

 These have the valves hinged along a straight line, 

 in the middle of which is a notch for the peduncle, 

 and the valves are often marked with ribs or striae. 

 The Orthids were content with limestone for their 

 shells, and apparently lived in the same circumstances 

 with the Lingulas; and in the period succeeding the 

 Primordial they became far more abundant. Yet 

 they perished at an early stage of the world's pro- 

 gress, and have no representatives in the modern 

 seas. 



In many parts of the Primordial ocean the muddy 

 bottom swarmed with crustaceans, relatives of our 

 shrimps and lobsters, but of a form which differs so 

 much from these modern shell-fishes that the question 



