1530 CRUSTACEA. 



The table shows that the torrid species, in none of the groups, 

 average larger than the extra-torrid. The cold-water Palinuridse are 

 as large as the largest warm-water species, and will outweigh them ; 

 the cold-water Galatheidea, are ten times the average length of the 

 warm-water; the Alpheinse, Palsemoninse, and Penseidse are at least 

 as large in the temperate regions as in the torrid. There is hence 

 nothing in the tropics to balance the Astacidse, a group of lar^e 

 species, some of them gigantic ; nor the Crangonidse, nor Pandalinse. 

 The genus Palaemon, in the Torrid zone, averages larger than in the 

 Temperate, the ratio being 3-5 to 2*40; the former amount being 

 reduced to 2-3 for the Palsemoninse, by the species of the other 

 tropical genera, which are mostly quite small. Yet, taking the ratio 

 of 3*5 to 2-40, it affects but little the balance against the Torrid zone. 



As to bulk, also, the Temperate zone probably has the preponde- 

 rance ; yet our data are less definite. In the Galatheidea, the cold- 

 water species are not only ten times larger lineally (which implies 

 at least eight hundred times cubically), but they are far more prolific, 

 swarming in vast numbers where they occur. The Thalassinidea are 

 more numerous in extra-torrid species than torrid, as well as larger in 

 size. The Scyllaridae are mainly tropical; but the species are not 

 of common occurrence, compared with the Astacidse, which abound 

 everywhere, and these, as well as the Crangonidse and Pandalinse, are 

 all Temperate zone species. The Palsemoninse and Penseidse probably 

 preponderate in the tropics, and this may be also true of the Alpheinse. 

 Taking a general view of the whole, and considering the fact, that the 

 extra-torrid species rather outnumber the torrid, we believe that the 

 deduction above stated is correct. 



In the Tetradecapoda, the number of species, the number and diver- 

 sity of genera, the number of individuals, and the bulk, are all greater 

 in the extra-torrid seas than in the torrid, as has been explained on a 

 preceding page ; and this is especially true of the Amphipoda. 



The tendency to spinose forms among the species of the colder tem- 

 perate regions, or Frigid zone, has been remarked upon on page 1523, 

 as exemplified among the Gammaridea, the Crangonidse, Litjiodes, 

 and Maioids. 



2. DISTRIBUTION OF CRUSTACEA ACCORDING TO GEOGRAPHICAL 



PROVINCES. 



The following tables are presented, as embodying in a general way 



