Life in the Ancient Seas 311 



or more, but their heart-shaped shells, generally known 

 as "parchment shells," are often cast up by the waves. 

 They are like the shells of bivalve mollusks, but with 

 this notable difference : the valves in a clam or mussel, 

 for instance, will be found on examination to be not 

 exactly alike in shape ; in the brachiopod both valves are 

 similar. There is no need here to go into the internal 

 differences; suffice it to say that in the brachiopod these 

 are so great as to make It more nearly allied to the 

 worms than to the mollusks. When it flourished In Its 

 pristine vigor, its numbers became enormous; and It is 

 one of the commonest forms among the oldest fossil- 

 bearing rocks. To-day it Is nearly extinct. It would be 

 interesting to know why this comparatively weak and 

 inactive creature has survived throughout that stagger- 

 ing lapse of time, while the sturdy trilobite, whose ap- 

 pearance seems to be contemporaneous, died out these 

 many millions of years ago. But this, like many other 

 secrets of the sea, is too profound even for speculation. 

 Now, if, as we are led to believe, the evolution of the 

 lower forms preceded in point of time that of the 

 higher forms, we should expect to find evidence of this 

 in the rocks. Well, that is precisely what we do find. 

 It is not until we reach the rocks merging Into those 

 which were formed In the period just preceding the 

 creation of the coal beds that we come across the first 

 fish. But this fish is far from looking like any that 

 exists to-day. In general shape it resembles that of a 

 tadpole rather than that of any modern fish. Its 

 comparatively great head Is depressed, projects out 

 on the sides, and is attached to a slender body the 

 length of which Is not more than twice that of the 



