Of the fourth group we have five specimens. The _Triaenopus_, so called
from its resemblance to a trident, has besides three leptodactylous
toes pointing forwards, a fourth extending backwards in a remarkable
way, like the handle of a trident; the impression, however, being
expanded so as to show an extensive displacement of the mud. All the
specimens of Triaenopus are in a beautiful red shale, very thin and
fragile, but presenting well-defined impressions, generally about
three inches long.
There are two species to this genus. Of the TRIAENOPUS EMMONSIANUS we
notice three impressions in relief. In another specimen there is the
appearance of a part of the toes of the Anomoepus Scambus, and on the
upper side are seen two excavations corresponding with the three
impressions. In the last slab, the track of the TRIAENOPUS BAILEYANUS
appears to have been made by two feet placed successively in the same
spot, which led President Hitchcock to suspect it might have been made
by a quadruped. One of the specimens has the Triaenopus tracks
intermixed in a peculiar way with other impressions.
The specimen representing the genus HARPEDACTYLUS is larger than the
preceding; and, though leptodactylous, the toes are much broader and
also more curved, whence the name Harpedactylus, _sickle-finger_, from
_harpe_ and _daktylos_.
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