Associate Professor Akira Takatsuki's Conjecture Vol 4

From the Yen Press edition

The truth behind Mana, the “Miracle Girl of Okutama,” comes to light! And soon after, the crew ventures out to investigate a mysterious cave said to enshrine an oni. There, they find not relics of a demon, but rather a human skull with a gaping hole... All this and more, as Professor Takatsuki and Naoya risk life and limb in their hunt for the supernatural!

Source: Yen Press