GREAT STEPS IN ORGANIC EVOLUTION 827 



may be thought of as the time when many, if not most, of the classes 

 of marine Invertebrates were estabUshed, such as sponges, some 

 corals, Graptolites, even jelly-fishes, starfishes, crinoids, trilobites, 

 and lamp-shells. It is in Cambrian rocks that we find the first 

 convincing evidence of the locking up of carbonate of lime in sub- 

 stantial skeletons, such as those of Crinoids — a process interesting 

 in itself and of great importance to the palaeontologist, since it 

 means the occurrence of well-preserved fossils. It was a habit that 

 grew upon certain animal types, for some that are still soft-shelled 

 in the Cambrian are very calcareous in the mid-Ordovician. 



Fig. 142. 



An Orthonectid, one of the simplest multicellular animals or Mesozoa. I, 

 ciliated outer layer; II, the internal mass of cell; M, an intermediate 

 layer. The small circles are the nuclei of the cells. 



Ordovician. — It is from mid-Ordovician (= Lower Silurian) 

 deposits that the first Chordate or Vertebrate fossils are known, but 

 the probability is that this great phylum began earlier ; perhaps, as 

 has been suggested, in a time of quick movement of water which 

 might serve as a stimulus to some vermine ancestors to move 

 energetically with, or (perhaps more promisefully) against the 

 stream. The earliest Chordate remains belong to the order of Ostra- 

 coderms, sluggish pre-fishes, probably off the main line of Vertebrate 

 advance. 



More impressive at the time must have been the Cuttlefishes or 

 Cephalopods, representing the highest class of Molluscs, and not in 

 any way related to true fishes. These Nautiloid Cephalopods are 

 first known from the Cambrian, but they became dominant in the 



