ACTINOMETRA IOWENSIS. 223 



Why this species, alone of all known, should have an ainbu- 

 lacral structure similar to that of Antedon and the Pentacrin- 

 idae, we cannot even conjecture. It is exceptional among 

 Actinomctrae for its very shallow habitat; but this does not 

 seem to suggest any explanation, since there are plenty of 

 Antedons and Actinometras living at equal depths in deep 

 water, which preserve their distinct ambulacral structures. It 

 is evidently to be treated as a transition or intermediate form 

 and it seems to me that we can only accept this as another of 

 those perplexing facts which are constantly coming to light 

 to show us how little we really know of the exact limits of 

 Nature's groups. 



