The orbitocranial fontanelle (FOC, Figures 5-9) of the interorbital septum shows a smaller aperture in Geococcyx (Figure 9) than in other Neomorphinae.
Geococcyx shows an elongated elevation on the mediocaudal surface of the upper jaw.
geoffroyi (Figure 8) and Geococcyx (Figures 7, 8, 9) this lamella is broader than in Tapera (Figures 1, 5) and Dromococcyx (Figure 6) because it covers the caudal portion of the nostril.
geoffroyi and Geococcyx (Figures 7-9) the choanal lamella (LC, Figures 5-9) is absent.
geoffroyi (Figure 8) and Geococcyx (Figure 9) the pterygoid is flat (the dorsal process is absent).
geoffroyi and Geococcyx (Figure 4) the lateral condyle is larger than the medial condyle.
Geococcyx and Neomorphus are very similar to each other, sharing most of the osteological features distinct from Morococcyx (see description).
In Geococcyx, Neomorphus and Morococcyx it is practically absent.
One of the few variations in this bone among species of Neomorphinae is found in Neomorphus geoffroyi, Morococcyx and Geococcyx, where the dorsal process is less developed and is partially fused with the mesethmoid, thus providing a large aperture for the medial orbitonasal foramen.
For example, in Neomorphus geoffroyi, Morococcyx erythropygus and Geococcyx (Figures 7 to 9) a thin plate of bone covers most of the nasal aperture, whereas in Tapera naevia and Dromococcyx phasianellus the nasal aperture is more open (Figures 5, 6).
c) Contrary to Hughes' (2000) observations, Tapera and Dromococcyx are similar to Neomorphus and Morococcyx (Figures 10-13) in the form of the crista basilaris transversa (characters 33, 34 of Hughes), as are all other cuckoos except Geococcyx (Figures 4, 14), Crotophaginae, Carpococcyx and some species of Coua and Centropus;
d) Although the elevation of the "bulbous" of os exoccipitale (character 37 of Hughes) is not prominent in Geococcyx and Neomorphus, in Morococcyx it is as salient as in Tapera and Dromococcyx and the Cuculinae species.