Monstrous Missions (anthology)

Monstrous Missions was a volume in the Doctor Who 2 in 1 books range, collecting Terrible Lizards and Horror of the Space Snakes.

Publisher's summary
One Time Travelling Doctor - Two Monstrous Missions

In Terrible Lizards, the Doctor and his companions join a group of explorers on a Victorian tramp steamer in the Florida Everglades. The explorers are searching for the Fountain of Youth, but neither they nor the treasure they seek are quite what they seem!

And in Horror of the Space Snakes, people are mysteriously disappearing on Moonbase Laika. They return with strange bite marks and no idea where they have been. Can the Doctor get to the bottom of what's going on?