The Appendages, Anatomy, and Relationships of TrilobitesRaymond, Percy E. (Percy Edward)
Science
The Appendages, Anatomy, and Relationships of Trilobites
Raymond, Percy E. (Percy Edward)
Trilobites
Viewed from the dorsal side, it is seen that the individual is very
imperfect, the greater part of the cephalon being removed by a
diagonal break which cuts off the anterior third of the left eye and
extends to the front of the second thoracic segment on the right side.
The ends of the pleura of both sides of the thorax are broken away, as
are also the greater parts of the pleural lobes and the posterior end
of the pygidium. On the ventral side, merely the posterior tips of the
hypostoma remain, but the distal ends of the appendages were so far
within the outer margin that the appendagiferous area is quite fully
retained.
The most conspicuous feature of this specimen is the presence of nine
pairs of large coxopodites behind the hypostoma, and of the remains of
ten pairs of endopodites, making in all ten pairs of appendages which
are easily seen. The apportionment of these segments to cephalon,
thorax, and pygidium is not agreed upon by the people who have
examined the specimens, but if one remembers that it is the outer
and not the inner end of the coxopodite which articulates with the
appendifer, it at once becomes evident that the first two pairs of
appendages on the specimen are the last two pairs belonging to the
cephalon, and that the next eight pairs are those of the thorax.
The impressions of fourteen pairs of coxopodites are readily counted
on the pygidium, and as Doctor Walcott noted sixteen pairs on the
actual specimens, his number was probably correct.
_Cephalon._
Projecting the line of the back of the cephalon through from the
dorsal side, it is found that the posterior tips of the hypostoma are
7 mm. in front of the posterior margin of the cephalon, and that the
points of attachment of the posterior pair of cephalic appendages
(the second pair shown on the specimen) are just within the posterior
margin. The gnathobases of this pair of appendages extend back some
distance beneath the thorax, and so give the impression that they
belong to that part of the body. So far as can be determined, the
cephalic appendages do not differ in any way from those of the thorax.
On the mould of the ventral surface, just outside of the lateral edge
of the right lobe of the hypostoma, is the somewhat imperfectly shown
impression of the endopodite of the third cephalic appendage. The
point of junction of the endopodite and coxopodite is about 2 mm. in
front of the tip of the adjacent branch of the hypostoma, and the
gnathobase is curved around just behind it. This accounts for three of
the pairs of cephalic appendages. The second cephalic appendages must
have thrust their gnathobases under the prongs of the hypostoma, and
the endopodites were probably close to its edge. No trace of this pair
appears on the specimen.
_Thorax._
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