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 Limtiliis 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 ' riTpov' 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 II. fig. 2, 

 ii-vii), I accept the homologies, and consequently adopt the terms applied to them 

 by Bell t, Woodward |, 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 antennae ' (ii in all the plates) of higher and 

 later Crustacea, is a forcipated limb, ditfering by its less number of joints and smaller 

 relative size from the succeeding forcipated pairs. It is interesting, also, to note that in 

 Scorpio, wliich, like Limnliis, goes back to the ' Coal-measures,' the corresponding ' anten- 

 nules * are forcipated. In lAmulus, however, the antennules are articulated by ' gom- 

 phosis ' to the sides of the base of a small ' labrum,' which is wedge-shaped, Avith the 

 edge below. 



Another analogy to Arachmda is exemplified in the ' outer antenna ' (iii, il).), or second 

 pair of limbs of Limulus, inasmuch as it is the seat of a sexual character. In the male of 

 Limulus imhiphemus it is monodactyle, the last joint being in shape a slightly bent claw 

 (PL IV. fig. 1, III). 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 II. fig. 2, iii). 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 (vii, ib.). This pair (PL II a. fig. 4) 

 has an additional joint {ib. 7). A long, slender, bi-articulate appendage (ib. r) is articu- 

 lated to the outer end of the hind border of the transversely extended haunch (ib. 1). 



* As, e.g., " siagonopod," " pereiopod," "pleopod," " uropod," &c. proposed by C. Spence Bate in the 'History 

 of British Sessile-eyed Crustacea,' part. i. p. 3 COctober 1861J. 

 t ' History of British Stalk-eyed Crustacea,' p. xx. (1853). 

 + ' Monograph on British Fossil Crustacea,' &c. 4to, 1866, p. 4. 

 § Van der Hoeven, tU supra, pi. i. fig. 3. 



