Geographical Distribution of Crustacea. 21 
of a series or chain of species—a fact illustrated in the chapter 
on the Classification of Crustacea. | 
Second, that the preponderance of cold-water species is the re- 
verse of what must have been true in the earlier geological epochs, 
when the oceans had a somewhat higher temperature; or were to 
a large extent tropical. 
T'hird, that the progress of creation as regards Crustacea, has 
ended 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.. À preponderance of species in the warmer seas is per- 
haps 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 mere- 
ly the cooler waters of the present tropics, but even the still colder 
temperature of the 'l'emperate zone, and therefore the present 
condition of the globe. 
''he genera of Fossil species commence with the Entomostra- 
eans and 'Trilobites in the Paleozoic rocks. | Next appear certain 
T'halassinidea and Astacoid species, in the Permian system ; then 
Mysidea, Peneidea, many T'halassinidea, Astacoidea, and Anom- 
oura,in the Oolitic. system ; then a few Cancroids and Leucv- 
soids in the Cretaceous, which become much more numerous 
in the 7'ertiary system, along with some Grapsoids. | None of 
the Ma?oids, the highest of Crustacea, have yet been reported 
from either of the Geological epochs. 
''he number of individuals and the s?ze are, for the Brachyura, 
greater in the '"l'orrid zone than in the colder regions. But for 
the Maeroura, the species of cold-water genera average nearly 
twice the lineal dimensions of those of warm waters; and the 
number of individuals also may possibly be greater. 
In stating the conclusion respecting the Macroura, on a pre- 
ceding page (last volume, p. 325), we omitted to give in detail the 
mean sizes of the different groups. "The following are the results, 
including the Galatheidea which are closely related to the Ma- 
croura :— 
Mean length of Mean length of 
'Torrid zone species. ^ Extra-torrid species. 
Galatheidea, : ; 0:3 inches. 3:0 inches. 
Thalassinidea, . : ie 9 € $0..9 
Scyllaridz, : : o0, 4 60 4 
Palinuride, . : TOC a 1590. 
Astacidae.—Homarus, à 140 * 
Astacine, . 4 qq-.4 
Nephrophing, . 50-2 
Crangonidze, : : : 20 *« 
Palzmonidz.— Alpheinse, Pv ." big" 
Pandaline, .. $0 * 
Palemonine, . 23 * 94 * 
Oplophorine, . 10 ^" 
Penzidz, e. irgqa." 4Ug 
