Otherwise, in animals that have attained approximately adult proportions--which appears to be at approximately 6 months of age in males--there are no variations which are ascribable to increasing age in the color-pattern or pelage that cause the systematist to confuse species or subspecies. Of the several parts of the skull in juvenal animals, the braincase and width of the posterior part of the palate are most nearly of the size attained in the adult, the facial part of the skull at birth is the least developed, and the interorbital region is, in relation to its ultimate adult size, intermediate in stage of development. The permanent teeth are acquired when the animal is approximately eleven weeks old. Four age groups, based on characters of the dentition and skull, have been recognized. They are: Juvenile.--One or more deciduous (milk) teeth present. Birth to three months of age. Young.--Sutures widely open between the maxillae and nasals and between the premaxillae and nasals. Three to seven and a half months of age. Subadult.--Sutures between maxillae and nasals visible but indistinct. Seven and a half to ten months of age. Adult.--Bones of rostrum coalesced with no traces of sutures visible to the naked eye. More than ten months old. The skull as a whole increases in size until the animal is two-thirds of the way through the stage designated as young. After this time the width of the rostrum, as measured across the hamular processes of the lacrimals, increases until approximately a third of the way through adulthood. The interorbital breadth decreases from late subadulthood to adulthood and even in adults there appears to be a slight decrease in this part of the skull with increasing age.