Answer:
Compound sentence.
Step-by-step explanation:
A compound sentence contains at least two independent clauses (with no dependent ones; if there is a dependent clause included, then it is a compound-complex sentence). There are two independent clauses in the sentence above: Its proprietor, Roderick usher, had been one of my boon companions in boyhood AND but many years had elapsed since our last meeting.
There are no dependent clauses here - both clauses are independent and can exist on their own.