_Juveniles._--The animals are born with a thick pelage of thin
aristiforms and thin setiforms. The color is uniformly blackish brown.
The nose, hands, feet, ears and tail are pinkish; P4 and M1 are already
erupted and the second molars are included in the bony alveoli. The
incisors are orthodont; the rostrum is short and the braincase is wide.
The posterior part of the skull is greatly curved dorsally. No change is
noticed in the pelage before the second molars erupt and become
functional.
_Adolescents._--As soon as the second molars become functional, the
pelage starts molting on the back. The thin aristiforms are still in
place but the aristiforms of the adult pelage may be noticed growing
under them in an oval patch which extends from behind the shoulders
caudad to the hips. At that age the first, agouti-colored aristiforms
appear on the mystacial region, immediately behind the vibrissiforms.
The rostrum gradually lengthens and the braincase appears to become less
inflated.
By the time the third molars erupt, the aristiforms start showing among
the setiforms which are now changing to agouti color in the same area on
the back, while the thin aristiforms of the juvenal stage disappear. The
agouti setiforms are appearing also posteriorly from the mystacial
region to the sides of the head and neck and, at the same time, on the
frontal region. The patch of glossy aristiforms and setiforms on the
back is sharply differentiated from the dull juvenal pelage of the sides
and rump. In a later stage the area of agouti setiforms on the sides of
the neck extends to the outer sides of the arms and finally reaches the
area on the back where the agouti setiforms were already developed.
_Adults._--When the third molars become functional, the agouti setiforms
are in place except for those on the upper sides of the neck. The
aristiforms have now extended over their normal area of distribution. As
soon as the third molars show wear, the premolars and first molars have
the counterfolds isolated in the occlusal surfaces as enamel islands.
Wearing gradually isolates all counterfolds.
_Senile_ individuals.--Progression of wear soon eliminates the signs of
the shallowest counterfolds from the occlusal surface and finally the
tooth is reduced to a short crown with a cuplike occlusal surface
completely filled with dentine. Only the main fold usually remains; it
is more or less reduced in size.
From the records of the Serviços de Estudos e Pesquisas sobre a Febre
Amarela, the following data for males of _Proechimys longicaudatus
roberti_ were obtained:
======================+=========+==========+========+=========+=======
| Number | | Length | | Length
AGE | of | Weight | of head| Length | of
| cheek- | in grams | and | of tail | hind
| teeth | | body | | foot
----------------------+---------+----------+--------+---------+-------
One day old | 2 | 20.5 | 70 | 53 | 24
17 days old | 2 | 26.0 | 110 | 60 | 28
Juvenile | 2 | 85.0 | 150 | 105 | 39
Adolescent | 3 | 115.0 | 175 | 120 | 45
Adolescent (older) | 3 | 180.0 | 195 | 135 | 45
Adult | 4 | 200.0 | 223 | 158 | 48
Senile individual | 4 | 360.0 | 230 | 170 | 48
----------------------+---------+----------+--------+---------+-------
The weights and measurements (except for one- and 17-day-old animals)
represent averages of specimens of the different ages.