376 T. D. A. COCKERELL. 



For the means of obtaining the photographic figures (the work 

 of Mr. T. C. Black) I am indebted to a grant from the American 

 Association for the Advancement of Science. The drawings 

 are by Miss Evelyn V. Moore. In this paper I have described 

 and figured only scales of fresh-water species. At some future 

 time, it may be possible to give a similar account of the marine 

 families, and of several fresh-water groups now omitted. 



