14 Geographical Distribution of Crustacea. 
The genera of Fossil species commence with the Entomostra- 
cans and Trilobites in the Paleozoic rocks. Next appear certain 
Thalassinidea and Astacoid species, in the Permian system ; then 
Mysidea, Pencidea, many Thalassinidea, Astacoidea, and Anom- 
oura, in the Oolitic system; then a few Cancroids and Leuco- 
soids in the Cretaceous, which become much more numerous 
in the Tertiary system, along with some G'rapsoids. None of 
the Maioids, the highest of Crustacea, have yet been “aim gabe 
from either of the Geological epochs. 
The number pbilidividaals and the size are, for the pilieakigeiras 
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 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- 
eroura :— 
Mean length of Mean length of 
Torrid zone species. tra-torrid species. “ 
Galatheidea, ‘ # 0:3 inc ches, 8:0 inches. - 
3 lassinidea, . eR as oo. 
Scyllaridee, ‘ : ou: co 
Palinuride, . j +320: °* 150: * 
Astaci Homarus, ‘ 140.“ 
Astacine, . 3 hs 
Nephrophine, . oy 
Crangonid, i : 2 2 = 
Palzmonide.—Alpheinz, ie 2 Sah Ps. * 
Pandaline, . 30 « 
Palemonine, . 23 “ teat 
; Oplophorine, . 10 “ 
Penzide, - i 2 oe : 45 * 
The table shows that the torrid species, in none of the groups, 
average larger than the extra-torrid. The cold-water Palinuride 
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 Alpheine, Palemonine, and Pe- 
neide are at least as large i in the tem perate regions as in the torrrd. 
There is hence nothing i in the tropics to balance the Astacide, a 
group of large species, some of them gigantic ; nor the Crangonide, 
nor Pandaline. The genus Palemon, in the Torrid zone, averages 
larger than i in the Temperate, the ratio being 3-5 to 2-4; the for- 
mer aoe being reduced to 2-3 oe ne Palemonina, by the 
the other tropical genera are mostly quite small. 
Yet, ea ratio of 3:5 ced alec te little the balance 
