Thomas Norwood wrote, in his Address on the Negro,
His cranial formation will remain a fixed characteristic, and with it will continue the rapid thickening of the walls of the skull at the age of puberty, when his apparent intellectual equality with white children ceases, and the animal or cerebellum takes possession and continues through life. This thickness of the skull has always been known by the negro, as he has used it as a weapon of attack by butting for untold centuries.
In The Color Line: A Brief in Behalf of the Unborn, William Benjamin Smith listed the 14 chief points in which blacks were closer to apes than to real people, among them
(8) Exceedingly thick cranium, enabling the Negro to butt with the head and resist blows which would inevitably break any ordinary European's skull.
That same list was also regurgitated by Theodore Bilbo in his book Take Your Choice: Separation or Mongrelization. It also was brought up in Congress in 1909, when Senator William Milton introduced a bill to ban interracial marriage (see Betrayal of the Negro, p. 363).
In The Negro a Beast, Charles Carroll quotes Alexander Winchell as saying
The [Negro] skull is very thick and solid, and is often used for butting, as is the custom of rams.
But my favorite example of this stereotype comes from Gonzalo Fernandez de Oviedo, who described Native Americans as
naturally lazy and vicious, melancholic, cowardly, and in general a lying, shiftless people. Their marriages are not a sacrament but a sacrilege. They are idolatrous, libidinous and commit sodomy. Their chief desire is to eat, drink, worship heathen idols, and commit bestial obscenities. What could one expect from a people whose skulls are so thick and hard the Spanish had to take care in fighting not to strike on the head lest their swords be blunted.
From Race: The History of an Idea in America, p. 12