1852.] H 
These subtribes may be divided into families. 
Subtribe 1. Thalassinidea. 'Thissection, as Milne Edwards observes, includes 
two strongly marked divisions; oze, with only the ordinary thoracic branchiz, 
and a second with the addition of abZominal branchial appendages, as in the Squil- 
lide. "The former we name the- TAa/assinidea Eubranchiata, the latter, the 
Thalassinidea Anomobranchiata. 'The first group embraces three families, dif- 
fering strikingly in outer maxillipeds and abdomen, as explained beyond. The 
second contains only two genera, Calianidea, Edw., and Zsec, Guerin—the last 
name was changed by Edwards to Ca/ltanisea ; but as this word is so near Callia- 
nassa and Callianidea, a contraction to Cal/zsea would be preferable. 
Subtribe 2. Astacidea.—In this subtribe, we adopt De Haan's sections, except 
that we exclude the Megalopidea, and. we do not associate the Thalassinidea 
with the Astacidea. The sections or families are Segy//aride, Palinuride, 
Eryonide and Astacida. 
Leach in 1819 divided the old genus Astacus, naming the marine species 
(Homarus Ed.) Astacus, and the fresh water (Astacus, Ed«.) Potamobius. 
Edwards? division, of like character, now generally accepted, was not published 
till. 1837. Leach hence has the priority. But according to Leach, the name 
Astacus is appropriated, not to the typical part of the group; that includins the 
Astacus fluviatilis of old authors, or Cazcer Astacus of Linnzus, and which em- 
braces at the present time numerous species, but to that including the Cazcer 
Gammarus of. Linnzus, still but a small group. "There is hence much objection 
to the names of Leach, and moreover much confusion would now ensue from 
theiradoption. "There seems therefore to be sufficient reason for rejecting them, 
if it be of no weight that they have remained for 30 years unrecognised by 
British authors. "They are adopted in the Catalogue of British Crustacea of the 
- British Museum, published in 1850, but not in the general catalogue of 1817. 
Subtribe 3. Caridezg.—ln arranging the Caridea into groups, much stress is 
usually laid upon external form and length of beak. "The unimportance of these 
characters is inferrible from the fact that they involve no essential variations of 
structure. Moreover, in a single natural group we may find both the long and 
short beak. In the Crangon group, for instance, in. which the beak is usuall 
very short and the body depressed, we have a species with the beak and habit of 
a Hippolyte. - 
"There are other characters of more fundamental value; and these have been 
brought forward by De Haan. "The mandibles afford the distinctions alluded to. 
In one section they are very slender and are bent nearly at a right angle, with- 
out enlargement at the crown. In another they are very stout, and somewhat 
bent above with a broad dilated crown. In a third, they are stout, but not bent, 
and have a dentate summit. Ina fourth they have, in addition to a projecting 
lateral crown, a large summit process, which is often oblong and very prominent. 
These forms are characteristic of different sections of the Caridea. 
The fact that the mandibles bear a palpus or not is of much less importance ; 
for the portion of the mandible which is most essential to its functions is the 
erown. Among the Palemoninz, there are genera having a mandibular palpus, 
and others without one; while the two kinds in other respects are remarkably 
close in their relations. 'We have found moreover that in this group, the length 
of the palpus varies with the disjunction of the 2d and 3d flagella of the inner 
antenna. If these flagella are separate to their bases nearly, (as in Palaemon,) 
the palpus is long and 3-jointed; if united for some distance up, the palpus be- 
comes short and finally only 2-jointed (Palzmonella;) if united nearly or quite 
to their summits, there is no palpus.* 
In the arrangement of the zenera into families, the fact «Aether the 1st or 9d 
* In our genus Palzmonel!a, the palpus of the mandible is 2-jointed, and in J4a- 
chistia, which is close:y like Palemon in habit in some of its species, there is no 
palpus, asin the Pontonie; and thus the transition to the Pontonie from Palzemon 
' 1s exceedingly gradual. Harpilius and (Edipus (Pontonis of authors) fill up the in- 
terval between Anchistia and the true Pontonie. "They are all similarin having the 
2d pair of legs largest, and in other prominent characteristics. 
