In Ruminantia, the roots of the incisiviform teeth in the lower jaw are only partially enclosed in the bony alveolus. Only the lingual part of the alveolus continues in oral direction, the labial part being open at the anterior side, and occupied by fibrous tissue, which is elastic and compact. One look at the skull of a cow, sheep, goat or deer will suffice to convince anybody of the correctness of this statement. Closing its mouth, the animal will press downwards the incisiviform teeth in the lower jaw with the gum pad of the upper jaw, till the lingual side of the crown of the front teeth meets the gum pad. Curiously enough, this mobility of the incisiviform teeth in Ruminantia has never been explained in any handbook on dental anatomy and, in some works only, has been mentioned by the way. AITCHISON was the first author who stressed this point (P.Z.S. 116, p. 329—338, 1946).