OF THE AMERICAN KING-CRAB. 463 
If like considerations have led careful and conscientious describers to propose definite 
terms, giving an escape from wrong analogies suggested by those borrowed from verte- 
brate nomenclature, for the various appendages of the exoskeleton of Crustacea*, I venture 
to hope that the term *cephaletron' may meet with some acceptance as applied to the 
anterior division of the body in both Limulus and Arachnids, and that the term * thorac- 
etron' may have the same fortune in relation to the second division of the body. Both 
terms indicate the composite analogies with the three great divisions of the body in 
anatomy; neither of them indicates or infers an homology adverse to the general conclu- 
sions which the ablest students of recent and fossil Crustacea have arrived at and agreed 
upon. | 
. The Greek term *77pov' signifies a part of the abdomen; and a part of such cavity is 
associated with the *head' in the first division of the King-crab's body, and with the 
*thorax' in the second division. For the third division (c in all the plates) I willingly 
adopt Mr. Spence Bate's proposed term of ‘pleon, including therein the part he calls 
‘telson,’ the whole constituting the characteristic “tail-spine” of the present singular 
genus. ! 
For the six pairs of articulate limbs, or appendages, of the * cephaletron (Pl. XXXVII. 
fig. 2, rr-vi1), I accept the homologies, and consequently adopt the terms applied to 
them by Bell t, Woodward 1, and others. 
These appendages are interesting in the present ancient form of the crustaceous class 
through the small amount of differentiation to which they have been subject. The 
homologue of the * antennules' or “internal antennæ” (11 in all the plates) of higher and 
later Crustacea, is a forcipated limb, differing by its less number of joints and smaller 
relative size from the succeeding forcipated pairs. It is interesting, also, to note that in 
Scorpio, which, like Limulus, goes back to the * Coal-measures,’ the corresponding * anten- 
nules’ are forcipated. In Limulus, however, the antennules are articulated by ‘gom- 
phosis’ to the sides of the base of a small ‘labrum,’ which is wedge-shaped, with the 
edge below. ; 
Another analogy to Arachnida is exemplified in the * outer antenna’ (111, ¿b.), or second 
pair of limbs of Limulus, inasmuch as it is the seat of a sexual character. In the male of 
Limulus polyphemus it is monodactyle, the last joint being in shape a slightly bent claw 
(Pl. XXXVII. fig. 1, 111). In the male Limulus moluccanus both second and third 
pairs of limbs are so modified $. In the females of both species the corresponding limbs 
are forcipated (Pl. XXXVII. fig. 2, 111). In both sexes the limbs succeeding the first 
pair, besides the addition of two basal segments (ib. fig. 3, 1,2), have a marked increase 
of length, and go on more gradually lengthening to the sixth (vir, 1b.). This pair 
(PL XXXVII. fig. 4) has an additional joint (i5.:). A long, slender, bi-articulate 
appendage (ib. 7) is articulated to the outer end of the hind border of the transversely 
* As, e. g., “siagonopod,” * pereiopod," * pleopod," “ uropod," $e. proposed by our esteemed colleague C. Spence 
Bate in the * History of British Sessile-eyed Crustacea,” part. i. p. 3 (October 1861). 
+ «History of British Stalk-eyed Crustacea,’ p. xx. (1853). 
+ * Monograph on British Fossil Crustacea,’ &e. 4to, 1866, p. 4. 
$ Van der Hoeven, ut supra, pl. i. fig. 3. 
