264 
CRUSTACEA. 
dreaded. Their eggs are eaten in China. When these animals walk, 
their feet are not seen. Fossil specimens are found in certain strata 
of a moderate antiquity 
In some, the four anterior feet, at least in one of the sexes, are 
terminated by a single toe. 
But a single species of this division is known ; it is the Limu- 
lus heterodactylus, and is the type of the genus T achy f lens 
Leachf . I have seen it figured on Chinese vellums. 
In the others, the two anterior claws at most, are alone monodac- 
tyle. All the ambulatory feet are didactyle, at least in the females. 
This division is composed of several species, which, owing to the 
little attention that has been paid to the detailed form of their parts, 
to the differences resulting from sex and age, and from their peculiar 
localities, have not yet been characterized in a rigorous and com- 
parative manner. The common American Limulus for instance, 
Avhen young, is whitish, or of a light colour, and has six stout teeth 
along the Avhole ridge of the middle of the upper shell, and two 
others equally strong and pointed on each lateral ridge of the shield, 
or of the first piece of that shell ; while older specimens, sometimes 
more than a foot and a half in length, are of a deep brown colour, or 
almost blackish, their teeth, the middle ones especially, being almost 
obliterated. Here also the lateral margins of the second piece of the 
shell are marked with fine dentations, which are scarcely apparent 
or wanting in the former. 
We should consider as young individuals the him. Cyclops, 
Fab., and the L. Soiverbii, Leach, Zool. Miscell., LXXIV ; his 
L. tridentatus , and the L. albus, Bose. : and as older ones, my 
Limule des Moluques ; Monoculus polypheniit? , L. ; Clus., 
Exot., lib. VI, cap. xiv, p. 128; Rumph., Mus., XII, a, b, which 
I at first considered a distinct species, under the belief that these 
large individuals inhabited those islands exclusively. In all of 
them, or at all ages, the tail is somewhat shorter than the 
body, and triangular, the upper ridge finely denticulated and 
without any decided sulcus beneath. We will designate this 
species by the name of Limulus polyphemus. These latter 
characters will distinguish it from some others described by Dr. 
Leachf. 
FAMILY II. 
SIPHONOSTOMA. 
The Siphonostomae have no kind of jaws whatever. A sucker or 
siphon, sometimes external, and in the form of an acute inarticulated 
* Knorr, Monum. of the Deluge, I, pi. XIV; Desmar., Crust, fossil., XI, 6,7. 
It would seem from these figures that the lateral spines of the second piece of the 
shell, in lieu of spines, merely form smaller teeth articulated at base ; hut these arti- 
culations have perhaps disappeared. 
t This Limulus is perhaps the Kabulogani or Unkia of the Japanese, and repre- 
sents the constellation of Cancer on their primitive Zodiac. 
See Nouv. Diet. d’Hist. Nat. Ed. II. ; Desmar., Consul., p. 344 — 358. 
