An environmental scientist is monitoring the population of a vulnerable species of pond snail (Lymnaea stagnalis) in a newly restored wetland habitat to assess conservation efforts.

In the first sampling session, the scientist captures 120 snails, marks their shells with a water-resistant, non-toxic paint, and releases them back into the pond. Five days later, a second sample of 150 snails is captured, of which 18 are found to be marked.

Using the Lincoln Index, what is the estimated population size of the pond snails in this habitat?