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 Tachypleus 

 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, 

 when young, is whitish, or of a light colour, and has six stout teeth 

 along the whole 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 Lim. cyclops, 

 Fab., and the L. Sowerbii, Leach, Zool. MiscelL, LXXIV ; his 

 L. tridentatus, and the L. albus, Bosc. : and as older ones, my 

 Limule des Moluques ; Monoculus polyphemus, L. ; Clus., 

 Exot., lib. VI, cap. xiv, p. 128; Humph., 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 Siphonostomse 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 ; but these arti- 

 culations have perhaps disappeared. 



f This Limulus is perhaps the Kabutogani or Unkia of the Japanese, and repre- 

 sents the constellation of Cancer on their primitive Zodiac. 



J See Nouv. Diet. d'Hist. Nat. Ed. II. ; Desmar., Consid., p. 344 358. 



