THE PROSOMATIC SEGMENTS OF AMMOCGTES 291 
Eurypterid. The segment immediately in front of this is the next 
for consideration, viz. that corresponding to the chilarial appendages 
or metastoma ; and as the basal part of this pair of appendages was 
fused with the basal part of the operculum, the one cannot be dis- 
cussed without the other; therefore, the segment to which the lower 
lip belongs must be considered in connection with and not apart 
from the thyro-hyoid segments already dealt with. 
In Chapter V., p. 188, I stated that the supporting bars of the 
foremost mesosomatic segments, the thyro-hyoid segments, differed 
from the cartilaginous bars of the branchial segments, in that they 
were composed of muco-cartilage. Also in addition to the muco- 
cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in 
Ammoccetes which covers over the thyroid gland. 
Similarly in the prosomatic segments the skeletal bars are com- 
posed of muco-cartilage and the ventral plate of muco-cartilage 
continues forward as the plate of the lower lip. It is of special 
interest, in connection with the segments indicated by such support- 
ing structures, to find that this special tissue is entirely confined to 
the head-region, and disappears absolutely at transformation, thus indi- 
cating tlie ancestral nature of the segments marked out by its presence. 
This muco-cartilaginous skeleton is the key to the whole position, 
and requires, therefore, to be understood. It is of great importance, 
not only because it demonstrates the position of the segments in 
Ammocecetes which characterized its invertebrate ancestor, but also 
because it possesses a structure remarkably similar to that found 
in the head-plates of the most ancient fishes. For the present I will 
confine myself to the consideration of this muco-cartilaginous skeleton 
as evidence of the relationship of Ammoccetes to the Eurypterids, 
and in the next chapter will show how absolutely the same skeleton 
corresponds to that of the Cephalaspide, so that Ammoccetes is 
really a slightly modified Cephalaspid, the larval form of which was 
Eurypterid in character. 
In Chapter IV., Figs. 63, 64, I have given a representation of the 
ventral and dorsal views of an Ammoccetes cut in half horizontally. 
Such a section shows with great clearness the series of branchial 
appendages with their segmental muscles and cartilaginous bars 
which form the branchial segments innervated by the IXth and Xth 
nerves, according to my view of the branchial unit. As is seen (Fig. 
64 or 115), the skeletal bar of the hyoid or opercular appendage, 
