PvECILOPODA. 145 



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 heteroductylus, and is the type of the genus Tachypleus, 

 Leach(l). 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 pecu- 

 liar localities, have not yet been characterized in a rigorous and 

 comparative 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 al- 

 most obliterated. Here also the lateral margins of the second piece 

 of the shell are marked with fine dentations which are scarcely ap- 

 parent or wanting in the former. 



We should consider as young individuals the Lim. cyclops, 

 Fab., and the L. Soiverbii, Leach, Zool. Miscell., LXXIV; his 

 L. tridentatits, and the L. albns, Bosc: and as older ones, my 

 Limule des Moluques; Monoculus polyphemus, 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 Leach(2). 



shell, in lieu of spines, merely form smaller teeth articulated at base; but these ar- 

 ticulations have perhaps disappeared. 



(1) This Limulus is perhaps the Kabutogani or Unkia of the Japanese, and re- 

 presents the constellation of Cancer on their primitive Zodiac. 



(2) See Nouv. Diet. d'Hist. Nat. Ed. II; Desmar., Consid., p. 344358. 



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