119 Botany and. Zoology. 
superiority, is seen in the fact that the sides of the carapax fold under: 
and unite to the epistome, a peculiarity well shown in Scyllarus and: 
also, though less perfectly, in Astacus. Another mark of this superi- 
ority is observed in the absence or small size of the basal scale of the 
outer antennce—this scale existing in no Brachyura, and having a large 
size in the typical Macroura. The Astaci are the transition. species. 
between the Astacidea and the next division of Macroura, and in 
the genus Paranephrops of White, the antennary scale is quite large 5. 
the Astaci differ from all the Macroura of the following divisions in the 
transverse suture which crosses the carapax near its middle. 
The remaining Macroura differ from the Astacidea in both of the 
characters above-mentioned ; the carapax is free from the epistome, and 
the antennary scale is large. "They are naturally separated into two 
sections marked off by the extent of their divergence from the higher 
Crustacea, and their different degrees of cephalie inferiority. "The 
distribution alluded to is indicated by the position of the strong prehen- 
sile legs. In the Brachyura the anterior pair is uniformly the strong 
pair; and this uniformity through so extensive a group shows that the 
variations from it must be of importance in classification. "This pecu- 
liarity of the Brachyura is a consequence of the concentration of force in 
the anterior portion of the cephalothorax or the anterior nervous ganglia ; 
and the diffusion of this force posteriorly, which in different degrees 
rnarks the Macroura, is especially exhibited in the legs. 
We observe then that through a large part of the Macroura the strong 
prehensile legs are either the first or second pairs. 'lhese species aré 
all of a common grade; for the species having the first pair the larger 
are connected by so many transitions with those that have the. second 
pair the larger, that no line of demarcation can be drawn which. should 
make a grad division among them.  'hese are the Can1DEA.. 
Another group remains, in. which the stoutest prehensile. legs are 
those of the /A?rd pair, and the line between these species and the 
preceding in this respect is strongly drawn. "This peculiarity indi- 
cates a transfer of force, which pertained to the first pair in the Bra- 
chyura, and to the first or second in the typical Macroura, to a pair 
more posterior: giving the anterior part of the body a still lower char- 
acter. "l'hese, the PENJEIDEA constitute our third division or subtribe. 
With the Penzeidea should be included certain still lower species, ap- 
proaching the Mysis group, in which none of the legs are stout chelate, 
(Sergestes, &c.) whose whole structure indicates their inferior charac- 
ter, and the low state of the forces within. : 
The three grand divisions, AsraciDEA, CAnIDEA and PENJEIDEA, con- 
stituting the second series, thus mark three grades of rank among the 
Macroura. "The Thalassinidea are the aberrant species. 
These subiribes may be divided into families. 
Subtribe 1. Thalassinidea.—' This section, as Milne Edwards | ob- 
serves, includes two strongly marked divisions ; one, with only the or- 
dinary thoracie branchiz, and a second with the addition of abdominal 
branchia! appendages, as in the Squillide. "The former we name the 
Thalassinidea Eubranchiata, the latter, the TAhalassinidea Anomobran- 
chiata. 'lhe first group embraces three families, differing strikingly in 
outer maxillipeds and abdomen, as explained beyond. The second con- 
