OF THE AMERICAN KING-CRAB. 465 
ging-edge. The almost horizontal tract between the lateral ridges, bisected by the 
median ridge, is traversed by the two longitudinal depressions, which are rather nearer 
the median than the lateral ridges. "These depressions, with the median and lateral eleva- 
tions, give the trilobitic character to the carapace or dorsal wall of the cephaletron of 
Limulus. Each longitudinal ridge terminates by a short spine posteriorly. They are 
much produced in the carboniferous Limuloids, e. y. Prestwichia, as in some Trilobites, 
whence they have received the name of * genal,' or ‘cheek ”-spines (in Trinucleus, e. g.). 
The hind vertical part or border of the dorsal wall of the cephaletron is divided into a 
median transverse tract of equal extent with the fore border of the thoracetron therewith 
articulated, and two lateral oblique tracts passing outward and backward to the hinder 
angles of the cephaletron, which are usually spiked (Pl. XXXIX, A"). The transverse 
posterior tract is vertically thick, as if truncate. At the middle third, included by the 
hind, deepened ends of the lateral longitudinal dorsal furrows, the upper border rises 
like the slopes of a low roof on each side to the median longitudinal dorsal ridge, here 
raised into a spine. The border below this is arched for the reception of the fuleral 
levator-process of the thoracetron, which passes beneath the arch. The joints of the 
thoracetron with the cephaletron are at the straight lateral tracts on each side the convex 
process, entering the arch and at a lower level. Consequently a power tending to pro- 
tract or pull forward the process, being opposed by the joints or centres of motion below, 
raises the thoracetron upon those centres, and brings it to a line or to a level with the 
dorsal plane of the cephaletron, or even raises it to a slight angle therewith. At the 
lower part of the transverse hinder tract, outside the piers of the arch, are the pair of 
small crescentic pilose depressions (Pl. XX XVI. fig. 1, 4) corresponding with the origins 
of the entapophyses within. From the posterior spine (ib. 4), terminating the lateral 
ridge of the dorsal surface of the cephaletron, a lower ridge passes downward and out- 
ward, to be lost in the lateral tract of the hind border. The indentation or groove 
between the ridge and border is continued along the thick transverse hind part of the 
cephaletron, defining the portion below, which articulates with the thoracetron. In this 
groove is situated the entapophysial pit (4). To the lower part of the so-defined hind 
surface of the cephaletron is attached the opercular plate, or first coalesced pair of 
lamelliform limbs (Pls. XXXVII., XXX. vin. fig. 1, vir. I view the groove above 
described as a persistent indication of an originally separate segment. This segment, 
by the entapophysial pits above and the lamelliform appendages below, y to the 
category of ‘ thoracetral’ plates: it is cephaletral only by confluence. 
The general aspect of the cephaletron of Limulus reminds one of a oa spade- 
blade, or the blade of a saddler's knife. It gives forward a digging-edge, curving outward 
and backward to nearly twice the breadth of the following segment; so that this can be 
drawn along in the track delved out by the foremost one with least resistance. The hard 
chitine (Pl. XXXVII. fig. 2, f) of the arched upper surface of the cephaletron (b) meets 
a flat tract of the same material below (ib. c), at an acute angle, to form the digging- 
edge (d) ; and this edge is strengthened by a low ridge, like the carpenter’s * bead,’ 
running above it, along the extent where most resistance has to be overcome, the * bead ” 
subsiding or falling into the edge at the hinder angular spiked ends (Pls. XXXVI. and 
VOL. XXVIII. 3g 
