lxxxviii 



THE VOYAGE OF H.M.S. CHALLENGER. 



coast of South America, also off Marion Island in the South Indian Ocean, in the Arafura 

 .Sea, and near Japan. 



It appears to me difficult to believe that these are not merely variations of one 

 and the same species, and that if they were compelled to reside under similar local 

 conditions, the unimportant specific distinctions would be bridged by many intermediate 

 forms. 



Although separated widely in space the conditions under which these species exist 

 may in some respects approximate to each other. Thus the temperature at which they 

 have severally been recorded to live off Japan and the western coast of South America 

 do not differ widely, being 41°'l in the former and 35°"5 in the latter. Although 

 Nematocarcinus altus was taken at a still greater depth off the north-west coast of the 

 island of Celebes, and near to land, at a depth of 2150 fathoms, with a temperature 

 of 3 8° "9. that is at a temperature that is more commonly recorded at the depth of 

 600 fathoms. 



If we compare the specific characters of one group with those of the other, we 

 shall find but little difference beyond the number of teeth on the rostrum. Closer 

 examination with others will show that these teeth are generally smaller and more 

 crowded in the first and less so in the second where the rostral process is longer. 



All these distinctions are but slight in relation to the physical conditions which 

 appear to lead to the true features of specific character. 



No specimen of this genus has so far as we know been found fossil. 



The fresh-water genus Atya is very remarkable, both for its peculiarity of form and 

 for the distant localities in which it has been found. 



The American naturalist Randal described a small specimen which was taken in the 

 rivers or ponds of the island of Hawaii, under the name of Atyoida, and another species, 

 but very closely resembling it, was taken by the late Dr. Stimpson in the island of 

 Tahiti, whence numerous specimens were brought home in the Challenger collection. 

 A third has been taken in the rivers of Mexico by Saussure, and, lastly, from the river 

 Potimerim in South America. 



The older known species that have longer been associated with the genus are eight 

 in number, and are found in localities here tabulated: — 



. New Zealand. 

 . New Caledonia. 



Mexico and West Indies. 

 . New Zealand. 

 . New Caledonia. 



Mexico and "West Indies. 

 . New Zealand. 

 . Cape Verde Islands 



Associated with Atya sulcatipes was a specimen of Atya [Atyoida) serrata, and a 



