PREFACE. 
Tue following Revision of the crayfishes of the Northern hemisphere is 
based mainly upon the material in the Museum of Comparative Zoilogy. 
Through gifts from friends, and through a system of exchanges carried on 
during the past few years, the Museum now possesses all the known species 
of Astacus from Europe and Asia, together with all the American species of 
Astacus and Cambarus excepting Cambarus angustatus (Le Conte), C. Wieg- 
manu Krichs., and C. Mexicanus Erichs., and the doubtful species C. ma- 
niculatus (Le Conte), C. Stygius Bundy, C. Nebrascensis Girard, and Astacus 
Oreganus Randall. In this collection is included the chief part of the 
material used by Dr. Hagen in the preparation of his Monograph of the 
North American Astacidz, a work which forms the foundation of our knowl- 
edge of the crayfishes of this continent. One may form some notion of the 
importance of the material received since the publication of Hagen’s me- 
moir, from the fact that twenty new species of Cambarus are made known 
in the following pages,* while Dr. Hagen. described but ten unknown to 
previous authors. 
Besides the collection in the Museum of Comparative Zoilogy, I have 
examined these :— 
1. The collection in the United States National Museum at Washington. 
Next to that at Cambridge this is the richest collection of Astacide in the 
United States, and both have been much benefited by interchanges. All 
of the new material received at the National Museum during the prepa- 
ration of this Revision has been promptly sent to me for study. For this 
invaluable aid I am under great obligation to Prof. S. F. Baird, Director of 
* Since this was written, descriptions of the new species have been printed in the Proceedings of the 
American Academy of Arts and Sciences, Vol. XX. pp. 108-135, December, 1884. Such a course seemed 
desirable on account of delay in the publication of the complete memoir. 
