G'eographical Distribution of Crustacea. 17 
VIII. 'Those species of a genus that occur in the colder wa- 
ters, are often more firmly put together, and bear marks of supe- 
riority in their habits. 'l'he Amphithoe and Gammari of the 
tropies are lax and slender species, of small size compared with 
those of the colder seas. 
IX. 'There is à tendency in the colder waters to the develop- 
ment of spinous species. "'T'his fact is as true of the. Podoph- 
thalmia as of the 'l'etradecapoda. Among the former, there are the 
thorny ZLithodes, the numerous Maioids armed with spines, the 
Acanthodes ; while the Cancroids and. Grapsoids of the tropics 
are usually very smooth and often polished species. "'l'here are 
the spinous boreal Crangons, the species of which genus. in the 
warmer seas are without spines. Among the 'T'etradecapods, the 
boreal IYphimedis are often spinous or crested ; Acanthonotus and 
Dulichia are spinous genera. 'Tl'he same tendency is seen in the 
third pair of caudal stylets in some cold-water Gammari, which 
have the branches spinulous instead of furnished with a few mi- 
nute hairs like those of the tropics. 
'T'here are also some spinous Crustacea in the tropies, as the 
Palinuride, and species of Stenopus. $Such facts, however, do 
not lead to any modification of the previous remark; for the 
tendency observed is still a fact as regards the several genera men- 
Goned; moreover the spinous tropical species are few in number. 
VI. ENTOMOSTRACA. 
'The Entomostraca have been little studied out of the 'T'em- . 
perate zone, if we except the results of the author's labors. 'l'he 
described species of most of the families are, therefore, almost 
exclusively from the temperate regions, and we know little of 
the corresponding species or groups in the warmer seas. "The 
following table presents the number of known species of the tor- 
rid and extra-torrid zones, omitting the Lerneeoids :— 
TABLE [v. 
: Torrid zone. Extra-torrid zone. 
LoPHRHYROPODA. 
Cyclopoidea, . . " . ' 120 (o R8 
Daphnioidea, : : : . : 5 46 
Cyproidea, . . . . . 18 61 
PHYLLOPODA. 
Artemioidea, ; , . : : 0 10 
Apodoidea,  . : : : : 2 o 3 
Limnadioidea, . « ; , : 2 2 
Pacrroropa. 
Ergasiloidea, , : . . : 1 4 
Caligoidea, : . ; : 16 33 
Were we to leave out of view the researches of the author, 
the number of species and the proportion for the Cyclopoides, in- 
stead of 120 to 76, would be about 3:50, thus not only revers- 
ing the rato, but giving to the 'lemperate zone almost all the 
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