318 Geographical Distribution of Crustacea. 
with extended legs, it sometimes covers a breadth of eleven feet, 
and the anterior legs or arms are four feet long!* The species 
of the other genera are mostly among the larger of the Maioids, 
and have no mark of inferiority. Such are the species of Maia, 
Pisa, Libinia, Hurypodius, etc. 
But among the species of the warmer section, we find the On- 
cininea and Parthenopinea, both manifestly inferior in grade, the 
ormer approaching even the Anomoura, and the latter forming 
the passage of the Maioids to the Cancroids, as has been ex- 
plained. We observe also the Periceride and the Tychida, all 
very small species, excepting a few Periceree: the Menethii, ‘Tia- 
rinizw, and Acanthonyces, are examples of the group. In addition, 
there are the Mithracide, which although attaining a large size 
show their inferiority in their shorter epistome, shorter body, 
which is sometimes even transverse, and their spoon-shaped fin- 
gers. In the last character, the Chlorodinze among the Cancroids, 
similarly show their inferiority to the Xanthide. That this kind 
of finger is such a mark of inferiority is apparent from its dimin- 
ishing in many species as the adult size of the animal is attained, 
the tendency being towards producing the acuminated finger 
found in the highest grades. 
We are hence sustained in the conclusion that the Maioids of 
the Temperate zone are generally those that are highest in grade. 
It also shows the congeniality of cold waters to the Maioids, that 
the only Brachyuran peculiar to the Frigid zone is of this group. 
We refer to the Chionecetes opilio. ' ; 
VIL The Brachyura, therefore, although most numerous in the 
Torrid zone, do not reach in this zone their highest perfection. 
On the contrary, the Temperate zone or colder waters are the 
habitat of the highest species. Hence, as the Maioidea stand first 
among all Crustacea, the highest development of the class Crus- 
tacea takes place, not in the Torrid zone, the most profuse in life, 
but beyond the tropics and coral-reef seas, in the middle Tem- 
perate Regions. 
VIII. The prevalence also of the inferior Corystoids in the 
colder waters does not invalidate this conclusion, as the fact re- 
specting the Maioids is wholly an independent one ; for these last, 
by attaining their highest perfection in these coldest waters, de- 
termine the principle as regards themselves, the highest grade of 
Crustacea. ower grades occur also in the colder waters, and 
the laws governing their distribution demand separate study and 
consideration. 
. Passing a step below the Maioids, we come to the Can- 
croids, and these, with the exception of the lower Corystoid spe- 
cies, and only one-eighth of the rest, are Torrid zone species. 
* De Haan’s 
+ On the consts of Britain” the Canerwids (excluding the swimming epeces) a 
only half as numerous as the Maioids. 
