1979] 
Carpenter — Permian Insects from Oklahoma 
265 
the metathorax, 2.5 mm long and 4 mm wide. The abdomen (incom¬ 
plete) is 25 mm long and 2 mm wide. The other paratype, no. 5181, 
consists of two fore wings and one hind wing; the fore wings are 28 
mm long and 6.5 mm wide; the hind wing, 26 mm long and 6 mm 
wide. 
In addition, there are 11 other specimens apparently belonging to 
this species, all isolated wings. 
The wings of this insect differed from those of other Protereisma 
by their large size, slender form, nearly straight anterior and poste¬ 
rior margins and the longer costal brace. The species was only 
slightly larger than P. insigne Tillyard, from Elmo, but the latter 
had a much broader wing, with a strongly curved posterior margin. 
P. directum presumably had a wing spread as great as 70 mm, which 
is larger than that of most existing mayflies but much smaller than 
the Jurassic Ephemeropsis tristalis, which had a wing spread of 
about 90 mm. 
Specimen no. MCZ 5182 is of special interest because of the 
excellent preservation of some parts of the body. The thorax, 
although somewhat crushed, shows the individual tergites very 
clearly (figure 4). Previously described specimens of Protereisma 
from Elmo have shown that the metanotum, although smaller than 
the mesonotum, was very much larger than it is in existing mayflies; 
this is shown in the accompanying photograph of directum. The 
pronotum consisted of a broad plate 1.2 mm long and 4 mm wide, 
about the same width as the mesonotum. 
The serrations along the costal margins of the fore and hind wings 
of Protereisma were described by Tillyard in 1932. They are clearly 
visible on the specimens from Midco, especially those of directum. 
Tillyard was apparently not aware that the serrations were equally 
well developed or even more strongly developed on the hind mar¬ 
gins of the wings of Protereisma. They are especially well preserved 
in the neotype of Protereisma latum Sellards, from Elmo (specimen 
MCZ 3419), and I take this opportunity to include two photographs 
(figure 5) of that specimen here, one showing the serrations along 
the costal margin and the other, those along the hind margin. The 
former also shows the setal bases on some of the veins. The serrated 
margins and setae on the veins are unknown in existing Ephemerop- 
tera, but they were well developed in the extinct Palaeozoic orders 
Palaeodictyoptera, Megasecoptera, Diaphanopterodea, and Pro- 
