ON THE 



GEOGRAPHICAL DISTRIBUTION 



CRUSTACEA. 



I. PRELIMINARY CONSIDERATIONS ON THE TEMPERATURE 

 OF THE OCEANS. 



THE temperature of the waters is well known to be one of the most 

 influential causes limiting the distribution of marine species of life. 

 Before therefore we can make any intelligent comparison of the Crus- 

 tacea of different regions, it is necessary to have some clear idea of 

 the distribution of temperature in the surface waters of the several 

 oceans; and, if we could add also, the results of observations at 

 various depths beneath the surface, it would enable us still more per- 

 fectly to comprehend the subject. The surface temperature has of 

 late years been quite extensively ascertained, and the lines of equal 

 temperature may be drawn with considerable accuracy. But in the 

 latter branch of thermometric investigation almost everything yet 

 remains to be done : there are scattering observations, but none of a 

 systematic character, followed through each season of the year. 



The Map which we have introduced in illustration of this subject, 

 presents a series of lines of equal surface temperature of the oceans. 

 The lines are isocheimal lines, or, more properly, isocrymal lines ; and 

 where they pass, each exhibits the mean temperature of the waters 

 along its course for the coldest thirty consecutive days of the year. 



