12 LINDSTRÖM, ON THECOCYATHUS NATHORSTI. 



the want of sufficient material prevents ray executing this. It 

 seenis, however, that the presence of a so called epitheca in the 

 former genus is too little important for a distinction. There may be 

 other characteristics, as the more developed pali in Trocho- 

 cyathus. 



The most remarkable feature in such Thecocyathi as Th. 

 mactra and Nathorsti is the great change to which they have 

 been subject: at an earlier stage resembling a Deltocyathus or 

 Brachycyathus and then assuraing the calicular form character- 

 istic to the genus. It may be, that this is a phenomenon indi- 

 cating the ancestry of these corals. 



