558 KNOWER. [Vol. XVI. 



study of some additional and better material have rendered the 

 present revision advisable. I have hence included a consid- 

 eration of two recent papers, that of Heymons (15) and that 

 of Willey (29). It is a pleasure to here thank Professor 

 C. O. Whitman for many courtesies extended to me during 

 two summers' work at the Woods Holl Marine Biological 

 Laboratory. 



August 16, 1899. 



