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
This is a practically entire specimen, on two blocks, one showing the
interior of the shell, and the other the one figured by Walcott, a
cast of the interior. The first shows the low rounded appendifers at
the anterior angle of each axial tergite. They are almost entirely
beneath the dorsal furrows and do not project so far into the axial
lobe as those of Ceraurus and _Calymene_. In fact, only those at the
anterior end of the thorax project inward at all. As expected, there
are five pairs on the pygidium. The cephalon is unfortunately so
exfoliated that the appendifers there are not preserved. The doublure
of the pygidium is extremely narrow.
The cast of the interior shows, rather faintly, the exopodites of the
right side of the thorax and of the left side of the cephalon, and,
still more faintly, the caudal rami and a few pygidial endopodites.
The exopodites on the right side are in what seems to be the customary
position, directed obliquely forward and outward, and the tips of
their distal lobes project slightly beyond the edge of the test. These
lobes were interpreted by Walcott as epipodites, but after comparing
them with the terminal lobes of the exopodites of specimens No. 65519
and 65514 I think there can be no doubt that they represent the same
structure. The pleura of the individual thoracic segments on this side
of the specimen have an unusual appearance, for they are bluntly
rounded or obtusely pointed, instead of being spinose.
The interpretation of the appendages of the cephalon is somewhat
difficult. At the left of the glabella there are two large exopodites,
the anterior of which lies over and partially conceals the other.
These show by their position that they belong to the fourth and fifth
cephalic appendages. In front of these lie two appendages which may be
either endopodites or exopodites, but which I am inclined to refer to
the latter. Both are narrow and shaped like endopodites, but bear on
their outer edges close-set fine setæ. They also show what might be
considered as faint traces of segmentation. If the first of these ran
under the end of the exopodite behind it, as shown in Walcott's figure
(pl. 22), then it would be necessary to interpret it as an endopodite,
but it really continues down between the exopodite and the glabella,
and seems to be attached opposite the middle of the eye. The specimen
does not indicate clearly whether this appendage is above or below
the exopodite behind it, but one's impression is that it is above, in
which case it also must be an exopodite. The appendage in front, being
similar, is similarly interpreted. If this be correct, then the
exopodites of the second and third cephalic appendages are much
shorter and narrower than those of the fourth and fifth. All of these
appendages are obviously out of position, for the cheek has been
pushed forward away from the thorax, though still pivoting on its
inner angle at the neck-ring, till the eye has been brought up to the
dorsal furrow.
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