18 



General Morphology. 



from which the other genera appear to radiate, but I must remain doubtful as to the true 

 positions of Duplorbis, Clistosaccus and Sylon. When the life-history and anatomy of Duplorbis 

 are better known it may be found necessary to remove it to a separate class, but the remaining 

 genera certainly constitute a natural group. 



Sylon 



Clistosaccus 

 Peltoyaster-like ancestor 



Duplorbis 



Peltogaster 



Parthenopea Sacculina Heterosaccus 



Triangulus Lernaeodiscus. 



In the above account only the essential features of the different genera have been touched 

 upon; a more exhaustive diagnosis will be found in the Systematic part, Chapter 7. 



