1488 CRUSTACEA. 



way. But the observations upon them have been made with less 

 thoroughness, and we have, therefore, confined our discussions to 

 Molluscs. 



Prof. Edward Forbes has with much discrimination laid down cer- 

 tain zones in depth, and pointed out their zoological and botanical 

 peculiarities for certain coasts. The observations on Crustacea made 

 by us, were not extended to any considerable depth, and they will 

 not enable us, therefore, to recognise these several zones in the follow- 

 ing tables. 



II. GEOGRAPHICAL DISTRIBUTION OF SPECIES. 



In making an application of the isothermal oceanic chart to the 

 subject of the geographical distribution of Crustacea, we have two 

 objects before us. 



First. — To compare the zones and their regions with one another 

 as to (a) number of species, (b) number of genera, (c) number and size 

 of individuals, (d) grade of species, in order to arrive at some general 

 conclusions as to the temperatures best fitted for the highest and most 

 prolific developments of Crustacea. 



Second. — To compare different geographical positions in similar 

 regions with one another, in order to arrive at their resemblances and 

 differences, and deduce the several distinct zoological provinces; and 

 also to distinguish the more or less wide diffusion of species in longi- 

 tudinal range. 



1. DISTRIBUTION OF CRUSTACEA WITH REFERENCE TO THE TEMPERATURE. 



We here present a series of tables, containing, for each genus, the 

 number of species that occurs in each temperature region, with a 

 column also giving the sum of the Torrid zone species, and another for 

 the sum of the Temperate zone species. The several regions are lettered 

 a, b, c, d, &c, to h, and where one or more species in a region occur 

 in another nearer the equator, it is indicated by annexing the number 

 with the letter of the column in which it occurs. Thus, 6 (2 a) in 

 column b, means that there are six species in the b or Subtorrid 

 Region, but two of them are found also in the a or Torrid Region. 



