As the tusks enlarged and the proboscis grew longer, the weight of the head and its appendages necessitated a largely increased area of attachment for the neck-muscles, and this was attained by a very great thickening of the cranial roof, the occiput not increasing proportionately; at the same time, the thickened bones were honeycombed with sinuses, so as to reduce their weight without sacrifice of strength. In those species of the Miocene _†Gomphotherium_ which had large and heavy tusks, this thickening was not very much less than in the true elephants. The enlargement of the tusks had other consequences, as, for example, in lengthening and broadening the premaxillaries and, in the elephants, in their downward bending, so as to shorten still further the facial region of the skull. With the development of the proboscis, the nasal bones were reduced to a minimum and the anterior nasal opening was no longer directed forward, but obliquely upward, while the nasal passage lost its horizontal direction and became almost vertical. The lower jaw continued to elongate the symphysis, reaching a maximum in certain species of _†Gomphotherium_; but the reverse process of shortening this anterior region of the jaw began with the reduction of the lower tusks, and, when these had disappeared, nothing remained of the immensely elongated symphysis, except the short spout of the elephant’s jaw. As the grinding teeth increased in height, there was a concomitant increase in the vertical depth of the jaws for their lodgment.