Pine/Grey Pine/Spruce Carpet (Thera firmata/obeliscata/britannica)

Pine Carpet (Thera firmata)
Inner edge of median line deeply indented (indicated by a)
Grey Pine Carpet (Thera obeliscata)
Compared to Pine, inner edge of median line forms an obtuse angle then line continues on a relatively straight course to the dorsum (shown by b left).
Spruce Carpet (Thera britannica)
Generally greyer in appearance than Grey Pine. Normally appears more variegated with veins showing strongly, creating appearance of obvious 'cells' within the median line (indicated by c). This line is often bordered by white.