CONTEXTS. 



Introduction, 1. 



History, 5. 



On the Constancy of the Specific Characters, 12. 



abnormal gigantic specimens, 13; accidental variations, 14; differences of age, 14; differ- 

 ent shape of the head, 14 : differences in sculpture, 15 ; hairiness, 15 ; older specimens, 15; 

 retrograde development, 15; postabdomen, 15; hands, 1C; colors, 1G; on the sexual dif- 

 ference, 17; the abdominal legs of the male, 17; the purpose of their structure, 18; the 

 females, 18; annulus ventralis, 19 ; ej^s, their attachment and bursting, 20 ; development 

 of the first pair of abdominal legs in the male, 20; second form of the male, 21 ; first form 

 of the male, 21 ; dimorphism or perhaps a sterile form, 22 ; inner sexual parts of the Cam- 

 bants male : of C. acutus, 22 ; of C. riri/is. 23 ; of C. Bartonii, 23 ; inner sexual parts of the 

 Cambarus female: of C. acutus, 24: of C. Bartonii, 24; no dimorphism in the true Astaei, 

 24; dimorphism in other Crustacea, 24 ; dimorphism in insects, 25; the rarity of varieties 

 an important character of Cambarus, 2G. 



Cambarus, 27. 



is Cambarus a distinct genus or not? 27; ernbryologieal confirmation as yet wanting, 29; 

 further division of the mentis Cambarus, 29 ; hooked legs of the male, 30; rostrum, 31 ; first 

 pair of abdominal legs, 32. 



Cambarus ; Description of the Genus and Species, 32. 



Group I., 32. 



Group II., 57. 



Group III., 74. 

 Astacus ; Description of the Genus and Species, 89. 

 On the Geographical Distribution, 97. 



distribution of the species, 9 7. 



distribution in every State or Territory, 99. 



number of species in the States and Territories as yet known, 103. 



our knowledge is at present very imperfect. 103. 



strict limitation of the habitat of Astacus and Cambarus, 104. 



limitation of Cambarus to North America and Cuba, 104. 



distribution of the three groups of Cambarus, 105. 



species associated, 106. 



geographical distribution according to the river systems, 107. 



distribution of the species, 1 09. 



number of the Astachlm known in the whole world, 109. 

 Explanation of the Plates, vii. 

 Index to the Species, 111. 



