THE PROSOMATIC SEGMENTS OF AMMOCGETES 295 
out these two segmental muco-cartilaginous bars and their attendant 
muscles, and see to what sort of segments their investigation 
leads. 
The bar which comes first for consideration (sks) arises imme- 
diately behind the auditory capsule from the first branchial cartilage 
very soon after it leaves the sub-chordal cartilaginous ligament; the 
soft cartilage of the sub-chordal ligament ceases abruptly in its 
extension along the notochord at the place where the hard cartilage 
of the parachordal joins it, and in a sense it may be said to leave the 
notochord at this place and pass into the basal part of the first branchial 
bar. The most anterior continuation of this branchial system is this 
muco-cartilaginous bar (sks), which passes forward and ventralwards, 
being separated from the axial line by the auditory capsule (cf. Fig. 
118, A, B, C). Its position is well seen in a sagittal section, such 
as Fig. 117. It follows absolutely the line of the pseudo-branchial 
groove (ps. br., Fig. 114), and ventrally joins the plate of muco- 
cartilage which covers the thyroid gland. It forms a thickened 
border to this plate anteriorly, just as the branchial cartilaginous 
bars border it posteriorly. In fact, it behaves with respect to the 
hyoid segment in a manner similar to the rest of the cartilaginous 
bars with respect to their respective segments. 
It represents, although composed of muco-cartilage, the cartila- 
ginous bar of the operculum in Limulus, which also forms the termi- 
nation of the branchial cartilaginous system, as fully explained in 
Chapter III.; it may therefore be called the opercular bar. 
The next bar (ska) is extremely interesting, as we are now out of 
the branchial or mesosomatic region, and into the region corresponding 
to the prosoma. It starts from a cartilaginous projection made of 
hard cartilage, just in front of the auditory capsule, called by Parker 
the ‘ pedicle of the pterygoid’—a projection ( ped.) which defines the 
posterior limit of the trabecule on each side, where they join on to 
the parachordals,—and winding round and below the auditory capsule, 
joins the opercular bar (¢/f. Fig. 118), to pass thence into and form part 
of the muco-cartilaginous plate of the lower lip. In the section figured 
(Fig. 116), this projection of hard cartilage is not directly continuous 
with (ska), owing to a slight curvature in the bar; the next few 
sections show clearly the connection between ( ped.) and (sk), and 
consequently the complete separation by means of this bar of the 
hyoid segment from the segment in front, In the figures, the hard 
