ORDER DECAPODA. 
183 
position of the eyes, and some other parts, Dr. Leach has es- 
tablished three genera: 1st. Scyllarus with the thorax as 
long or longer than broad, without lateral incisions, and the 
eyes always situated near its anterior angles. The last arti- 
culation but one of the two posterior feet is uni denticulated in 
the females. ( Cancer arctus , Linn.) 
2nd, His Then US has the thorax measured in front 
broader than long, with a deep incision at each lateral edge, 
and the eyes situated at its anterior angles. ( Themis Indians, 
Leach.) 
3rd, His IBACUS does not differ from thenus but in the po- 
sition of the eyes, which are approximated from the origin of 
the intermediate antennae, or much more interior. (. Ibacus 
Per on ii, Leach.) 
Palinurus, Fab., 
Have the lateral antennae large, setaceous, and bristling with 
prickles. Some of these Crustacea arrive to a very consider- 
able size. In the species of our coasts, and probably in 
others, the females have at the extremity of the last articulation 
but one of the two posterior feet, a projection in the form of a 
spur or tooth, exclusively proper to this sex. Scyllarus presents 
the same difference. (. Palinurus quadricornis , Fab.) 
The third section, that of Astactni, Lair is distin- 
guished from the preceding, by the form of the two anterior 
feet, and often by that of the two following pair, which termi- 
nate by a forceps with two biters, or a didactylous hand. In 
some the last two or four are much smaller than the preced- 
ing, which approximates them to the anomala. But the fan- 
like fin of the extremity of the tail, and other characters 
lemove them from these last. The thorax is narrowed in 
front, and the forehead advances more or less, in the manner 
of a beak or pointed muzzle. 
Some Galathadeas, Leach , have, as well as the preced- 
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