"This specimen is a short bifoliate stem with three rows of apertures on each flattened side and none on the edges. Slightly raised longitudinal ridges separate adjacent rows of apertures. Apparently no mesopores are present between apertures but they may have been destroyed by recrystallization" (17).
Fagerstrom, J.A. (1961). The fauna of the Middle Devonian Formosa Reef Limestone of southwestern Ontario. Journal of Paleontology 35(1):1-48.
There are some interesting branching, radiating patterns in this one, with two zooecial apertures near the upper left and upper right corner (the dimply stuff). Colony form here is likely remnant of bryozoan encrusting substrate (with thanks for our experts on the forum). But why are we even talking about Amherstburg Formation? Let's keep this flagged for the time being.
Consulting Ludvigsen's 1979 text, Fossils of Ontario. Part 1: The Trilobites, there is a specimen reported that looks nearly identical to this one, but it is simply called Dechenella halli. The name was updated by Ludvigsen in 1986 and recognized as a new genus: Mannopyge halli.
Here is a plate from the Ludvigsen 1986 text on the left, compared to my find on the right:
"A warburgelline with pear-shaped glabella, deep sigmoid 1s furrow, narrow (tr.) and faint 2s and 3s furrows; no preglabellar field, tropidium, or tropidial ridges. Large eyes located anterior of cephalic midlength; genal spines short. Semicircular pygidium iacks a flat border,-axis with 9 - 10 node-bearing rings, eight faint pleural furrows and incised interpleural furrows, each pygidial rib terminates abaxially as a rounded node isolated by moderately deep paradoublural furrow. [...] No other warburgelline has a semicircular pygidium, and none possesses a conspicuous row of fringing nodes such as that of Mannopyge. The pygidial pleural ribs of M. halli, however, are of the flat-topped warburgelline-type (Owens 1973, Fig. 2), and there is no reason to doubt that Mannopyge is a late member of the subfamily Warburgellinae." (Ludvigsen 1986, 683).
Ludvigsen, Rolf (1986). Reef trilobites from the Formosa Limestone (Lower Devonian) of southern Ontario. Canadian Journal of Earth Sciences (24): 676-88.
Two remarks: First, this tells me that there are some Amherstberg formation rocks in the mix at this site. Second, trilobites in the Formosa reef limestone are not particularly common, dominated as it is by coral and stromatoporoids. Of the uncommonly found trilobites in that limestone, it is mostly dominated by Crassiproetus, followed by frequency occurrence Mannopyge halli, followed - in descending order of frequency - by Mystrocephla, Acanthopyge, and Harpidella.
Ok, enough from me until next weekend, when I'll be headed to a quarry east of Lake Simcoe for some serious Ordovician collecting. Until then, thanks for reading!