TRIARTHRUS Green, 1832. Triarthrus eatoni (Hall) (x2.7). "Beecher's trilobite bed," Utica Shale, U. Ordovician, Rome, New York (YPM 218; loaned through courtesy of J. Cisne.) This and the following plates through plate 128 represent a unique record of Beecher's famous trilobites, which were preserved with most of the soft parts replaced by fine granules of iron pyrite. As a result, such external anatomical details as antennae and appendages can be easily seen, while X-ray photographs can detect details of the internal anatomy. The photograph presented in this plate was obtained after immersion of the shale slab in xylene. The antennae and appendages become clearly discernible by this approach. One of the specimens, originally prepared by Beecher, shows the ventral side of the trilobite.

Triarthrus eatoni (Hall) (x6.3) (YPM 218E). Same origin and disposition as plate 125. Photograph through immersion in xylene. The biramous appendages of the right-hand side project outside the exoskeleton.

Triarthrus eatoni (Hall) (x7.7) (YPM 204). Same origin and disposition as plate 125. An X-ray view (a) and a xylene immersion view (b) of the same specimen. The original radiograph was taken by John L. Cisne and kindly loaned to the author. The tesolution of the photograph obtained from the xylene-immefsed specimen enables the fine structure in the biramous appendages, patticulatly the gill btanches, to become apparent.

Triarthrus eatoni (Hall) (x4.1). Same origin as for plate 125. (Am. Mus. Nat. Hist. 839/14 A,B through courtesy of John L. Cisne. X-ray negative loaned by John L. Cisne.)

Family Asaphidae Burmeister, 1843

Subfamily Asaphinae Burmeister, 1843

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