Complex sentence: He organized a small school for the plantation employees, but devoted much his time to malacology, specifically the study of Hawaiian land snails, some of which he named and decribed.
Split into simpler sentences: He organized a small school for the plantation employees. At this time he devoted much of his efforts to studying mollusks, i.e. to malacology, specifically the study of Hawaiian land snails, some of which he named and decribed.