While collecting from the lower “Trenton” limestone, called the Salona formation by Professor Field3 in 1919, at the type locality, Salona, Clinton County, central Pennsylvania, in the summer of 1928, I obtained a large number of specimens of the trilobite described by Simpson4 forty years ago as Homalonotus trentonensis. Among the many dissociated parts were free checks and hypostomes, which had never been described, and one complete articulated cranidium, thorax, and pygidium.

Homalonotus trentonensis is of particular interest because it is the oldest Homalonotid known in North America. Its stratigraphic and geographic range is limited, and it appears to be closely related to one of the Ordovician species found in Wales.

It is the purpose of this paper to review the history of the species, to give a fuller and more accurate description of it in the light of this more complete information, and to compare it with its . . .

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