The shape and dimensions of the incisive foramen long have been
recognized as providing specific characters. Large size of the foramen
is probably correlated with the requirement for a large amount of
moisture reaching Jacobson's organ in the nasopalatine space; the
moistening of the sensory epithelium is certainly involved. There seems
to be a certain correlation between small size of the incisive foramen
and high degree of humidity in the environment. Shapes and dimensions of
the foramen appear as simple or multiple biotypes and provide characters
which can be employed to differentiate subspecies, species and even
subgenera. Usually a character, say a general shape, occurs in nearly
all populations of a given subspecies but the particular shape seems to
be more closely correlated with ecological conditions, especially
humidity. Animals which live far away from large rivers usually have
larger foramina than animals which live close to rivers.
Both the premaxilla and the maxilla develop processes which form a
sheath for the vomer. This vomerine sheath forms a bridge which
longitudinally crosses the incisive foramen; the structure of this
bridge varies widely. Sometimes the maxillary part is not developed and
the sheath is incomplete posteriorly; sometimes this maxillary part is
very slender and merely touches the premaxillary part. The premaxillary
part, however, is always well developed.