GEOGRAPHICAL DISTRIBUTION OF CRUSTACEA. 1529 



not where it begun, in multiplying the species of warmer waters and 

 giving them there their superior developments, but in carrying 

 species to a higher perfection in the colder regions of the oceans. A 

 preponderance of species in the warmer seas is perhaps to be expected, 

 since warm waters have prevailed even more largely than now in 

 earlier epochs. But it would seem, that the introduction of the higher 

 grades of Crustacea required, not merely the cooler waters of the 

 present tropics, but even the still colder temperature of the Temperate 

 zone, and therefore the present condition of the globe. 



The genera of Fossil species commence with the Entomostracans 

 and Trilobites in the Palaeozoic rocks. Next appear certain Thalas- 

 sinidea and Astacoid species, in the Permian system; then Mysidea, 

 Penceidea, many Thalassinidea, Astacoidea, and Anomoura, in the 

 Oolitic system; then a few Cancroids and Leucosoids in the Creta- 

 ceous, which become much more numerous in the Tertiary system, 

 along with some Grapsoids. None of the Maioids, the highest of 

 Crustacea, have yet been reported from either of the Geological epochs. 



The number of individuals and the size are, for the Brachyura, 

 greater in the Torrid zone than in the colder regions. But for the 

 Macroura, the species of cold-water genera average nearly twice the 

 lineal dimensions of those of warm waters; and the number of indi- 

 viduals also may possibly be greater. 



In stating the conclusion respecting the Macroura, on a preceding 

 page (p. 1515), we omitted to give in detail the mean sizes of the 

 different groups. The following are the results, including the Gala- 

 theidea, which are closely related to the Macroura : — 



Galatheidea, . . , 



Thalassinidea, 

 Scyllaridge, 



Palinuridae, .... 

 Astacidae. — Homarus, 

 Astacinae, 

 Nephropinae, 

 Crangonidae, 

 Palaenionidae. — Alpheinae, 



Pandalinae, . 

 Palaemoninae, 

 Oplophorinae, . 1-0 " 



Penseidse, . . . . 3-6 " 4-5 



383 



Mean length of 

 Torrid zone species. 



0-3 inches. 



Mean length of 

 Extra-torrid species. 



3-0 inches. 



2-0 



(i 



30 



u 



60 



a 



60 



U 



12 



u 



150 



u 







140 



H 







30 



u 







50 



a 







20 



f< 



1-5 



it 



1-5 



a 







30 



u 



2-3 



u 



2-4 



a 



