Walcott figured a slice of Isotelus gigas from Trenton Falls, New York, which shows a few fragments of appendages, but is of particular importance because it shows the presence of well developed appendifers beneath the axial lobe.
Illustrated: Ottawa Nat, vol. 24, 1910, p. 129, pl. 2, fig. 5.
The following quotations from my paper are inserted here to complete the record of appendage-bearing specimens:
A rather remarkable specimen of this species was found by W. C. King, Esq., on the shore of Lake Deschenes at Britannia [near Ottawa, Ontario]. This specimen is an impression of the lower surface of the trilobite, and shows a longitudinal ridge corresponding to the central furrow along the axis of the ventral side of the animal, ten pairs of transverse furrows, and the impression of the hypostoma. The doublure of the pygidium has also left a wide smooth impression, but in the cephalic region the hypostoma is the only portion of which there are any traces remaining. The specimen was found on a waterworn surface of the beach, partially covered by shingle….
The transverse furrows are the impressions left by the gnathobases of the basal joints of the legs. They were evidently long and very heavy, but the specimen has been so abraded that all details are obscured. The first six pairs of impressions are longer and deeper than the four behind. The first eight pairs seem to pertain to the thoracic appendages, while the last two belong to the pygidium. From the posterior tips of the hypostoma to the first gnathobases of which traces are present there is a distance of about 22 mm. without impressions. In Isotelus gigas the hypostoma normally extends back to the posterior margin of the cephalon, so that it seems that in this specimen the impressions of the first two pairs of gnathobases under the thorax may not have been preserved. In that case, the six pairs of strong impressions may represent the last six pairs of thoracic segments, and the pygidium might begin with the first of the fainter ones.
Horizon and locality: From the sandstone near the base of the Aylmer (Upper Chazy) formation at Britannia, west of Ottawa, Ontario. Specimen in the Victoria Memorial Museum, Geological Survey of Canada, Ottawa.
The Appendages of Triarthrus.
(Pls. [1]-[5]; pl. [6, figs. 1-3]; text figs. [1], [10], [11], [33], [42].)