218 BRITISH FOSSIL CRUSTACEA.. 



becomes flatter and expands laterally ; the cephalic shield increases in breadth, and the 

 hinder body is curved upon the cephalon, the larva assuming the position of an Amphipod 

 in the egg, with its dorsal surface convex instead of its ventral surface, which is now 

 concave. This stage, described by Packard, may probably be that figured by Dr. Dohrn, 

 and copied by us from his memoir (see PI. XXXIII, fig. 9). In this figure the differen- 

 tiation of the articuli of the limbs has proceeded further than in fig. 4, the opercular and 

 first and second thoracic branchial plates [op., dr.) and the rudimentary ' metastomata ' or 

 ' chilaria ' (c/i.) are also discernible, and the chelae some free, others still apparently 

 enclosed at their ends in the simple finger-tips of the previous moult. 



Lastly, just before hatching, what Dr. Packard calls " the trilobitic shape " (PI. 

 XXXIII, figs. 6, 7, and 8) is assumed. This stage is peculiarly interesting to the 

 palaeontologist as well as to the embryologist as illustrating some points in the structure 

 of the extinct members of the order not seen in the fully developed living King-crabs. 

 Thus, in the deeply-emarginated head-shield of young Limulus at this stage, one is led to 

 recall the indication of segmentation in the cephalon of Hendaspis (see Woodcut, figs. 64 

 and 68, p. 178). We are also furnished with a means of interpreting the curious punctate 

 or digitate structure of the margin of the head-shield in Trinucleus concentricus (see PL 



XXXII, figs. 38 — 41). The late Mr. J. W. Salter 1 supposed that the membranous margin 

 of the head-shield of Trinucleus was once entire, " then became plicate, then perforate, 

 and lastly separated into linear processes" (fig. 40 /). It appears to us more in accord- 

 ance with the observed mode of evolution of the head- shield in larval Limulus (PI. 



XXXIII, figs. 4, 6, 7, 12) to conclude that the margin of the head-shield in Trinucleus 

 was originally digitate, then gradually closed up, leaving only perforations along the 

 sutures in some, and only plicae in others. 



We have an analogous modification of the shells of certain Mollusca, e.g., Haliotis, 

 Scissurella, and Pleurotomaria, in which a slit becomes partially or wholly closed, leaving 

 a furrow with perforations at intervals. It must be borne in mind that in these 

 Gasteropods the structure referred to is connected with the respiratory functions, but in 

 Limulus, Llemiaspis, and Trinucleus it indicates the margins of the primitive segments 

 which have coalesced to form the cephalic shield. 2 



This trilobite stage of larval Limulus proves, if further evidence were wanting, that the 

 front shield of Limulus is the head (cephalon), and not the cephalothoraw (as erroneously 

 called by Dr. Packard). In this view I am in accord with Professors Owen and Huxley, 

 Mr. C. Spence-Bate, and other carcinologists. 3 



In like manner the hinder-body of the young King-crab (PI. XXXIII, figs. 6 and 

 12) at this period serves admirably to illustrate the adult Limuli of palaeozoic times. 



1 'Quart. Journ. Geol. Soc.,' vol. iii, 1847, p. 2.i2. 



2 H. "Woodward, article " Crustacea," ' Encyclop. Brit.,' 9th edition, 1877, vol. vi, p. 650. 



3 See " Further Remarks on the Relationship of the Xiphosura to the Eurypterida and to the 

 Trilobitaand Arachnida/' by H. Woodward, ' Quart. Journ. Geol. Soc.,' 1872, vol. xxviii, pp. 46 — 63. 



