_Body_, slightly convex; contractile?
_Buckler?_
_Abdomen_, with three articulations, side lobes longitudinal, narrow,
and wedge-shaped.
_Tail_, broad, rounded, without any membranaceous expansion.
The name of this genus is derived from the circumstance, that the
abdomen has but _three_ articulations; an organization which is very
peculiar. These curious fossil animals are very abundant in the rocks
in which they are found; but though I have examined a multitude of
specimens from different localities, no vestige of the head or buckler
could, on the most minute examination, be discovered. Whether these
animals, during their petrifaction, were so contorted or rolled up,
as to bring the extremities of the body together, in such a manner as
to present the posterior folded part only to the view; or whether the
buckler has been destroyed by the process of mineralization, as appears
frequently to happen with the asaphs, we are at a loss to determine.
The animal remains which belong to the genus Triarthrus, differ so much
in their' form and general characters from all the other trilobites,
that we perhaps ought to regard them as forming another race of beings.
They are, however, more nearly allied to this family than the Agnosti
of Professor Brongniart.
Triarthrus Beckii. _Green._ Cast No. 34. Fig. 6.
Cauda subrotunda, bipunctata; articulis abdominis tribus, absque
lobis lateralibus consuetis, sed lobo arcuato utrinque apposito.
The only portions of this fossil which have yet been found, are the
abdomen and tail. The abdomen is composed of three joints; the first
passes from the side lobes completely over the body, and on its upper
surface, near the middle of the back, there is often a minute elevated
pimple. The other two, pass obliquely from the lateral lobes, and are
interrupted in their course over the body. The tail, or posterior
portion, is expanded, something like that of the Isotelus or Dipleura,
and has a deep puncture on each side, about half the distance between
its terminal border and the last articulation of the abdomen. The
lateral lobes are unlike those of any other genus. They form narrow
cuneiform appendages to the sides; near the first joint of the abdomen
they are crossed transversely by an elevated ridge, from which they
gradually taper along the sides of the body, and appear to inosculate
in a delicate point at the central border of the tail. The abdominal
articulations do not pass over these lobes, but just below the last
joint, a little transverse furrow, in perfect specimens, may be
noticed. The largest specimen of this fragment I have seen is exactly
half an inch in length.
This fossil occurs in black shaly limestone, on the canal near Cahooes
Falls, in the State of New York, and at a number of other places in
that State.
I have named this species in compliment to my early friend, Professor
Theodore Romeyn Beck, M. D., well known both at home and abroad, as
the learned author of the work on Medical Jurisprudence. Some time
after commencing this little Monograph, I communicated my plan to Dr.
Beck, and was surprised and gratified to find that he was also engaged
with the same inquiries, and that he was then busy in arranging and
examining the unique collection of trilobites belonging to the Albany
Institute. Without the smallest hesitation, he placed all his specimens
at my disposal, and has facilitated otherwise my undertaking, by every
means in his power.