PREFACE. 



THE following Revision of the crayfishes of the Northern hemisphere is 

 based mainly upon the material in the Museum of Comparative Zoology. 

 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 angustatiis (Le Conte), C. Wicg- 

 manni Erichs., and C. Mexicanus Erichs., and the doubtful species C. ma- 

 mcuMus (Le Conte), C. Stygius Bundy, C. Nebrascensis Girard, and Asiacus 

 Orcgamis 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 Astacida?, 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 Zoology, 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 Astacidte 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, 1834. Such a course seemed 

 desirable on account of delay iu the publication of the complete memoir. 



