Journal of the Federated Malay States museums - Sabrizain.org

8 Journal of the F.M.S. Museums. [Vol. VII,
(2) There is probably a small error due to blacklash or
spting in the dividers; this error is positive and
is relatively greater in the smaller than in the
larger measurements.
(3) In the case of the measurement of total length old
skulls may give a longer measurement than that
really representing their morphological size due
to the development of post-occipital ridges. The
length is also increased at the anterior extremity
in very old specimens owing to occasional
ossification of the cartilage at the tips of the
nasals.
In the case of the measurement of the length of the
nasals an element of uncertainty is often introduced by the
irregularity of the suture with the frontal. This error may be
either positive or negative.
In the Zygomatic breadth, a negative (i.e. the measurement
obtained is too small) error is introduced by the spring
of the zygomatic arches.
The Diastema is affected by the position of the roots ot
the anterior premolars which spread forward to a variable
degree. This error also is negative.
The tooth-row measurement, which is taken on the
alveolus, is affected in the same way, though the error in this
case is positive (i.e. the result is too large); and also in old
skulls by actual absorption of the teeth when the sign is
negative. This is not very marked in most rats* but the genus
Rhinoscinrus (Sciuridae) may be cited as an extreme case.
The specimens which have been measured, have been
selected as adult, those specimens which show no signs
whatever of wear on the molars having been rejected.
The arithmetic mean error, the error of mean square or
Standard Deviation of Pearson have been calculated as also
the Coefficient of Variation. It will be noted that the
measurements in all cases, if plotted, form curves of a
symmetrical type, the arithmetic mean agreeing very closely
with the Median.
In the case of the upper tooth-row I have not given the
standard deviation or the arithmetic mean error as the
measurement does not admit of sufficient accuracy to give
consistent results, the actual dimensions being very small and
the normal variation being apparently contained within very
narrow limits.
Table I. Measurements of Epimys ratttts jarah
(Bonhote)
Adult males.
Table II. Do.
Adult females.