The skull of the female oregonensis is more heavily ridged and is relatively broader across the mastoids than that of effera.
The slenderness is especially noticeable in the breadth across the zygomata, mastoids, and rostrum.
mastoid mas·toid (mās'toid')
The mastoid process. adj.
Of or relating to the mastoid process, antrum, or sinuses.
Shaped like a breast or nipple.