Latin sylva

This says Sylvan comes from Latin sylva, which I had never seen. I know silva, with an i, not a y, and I wondered why there should be a y there instead since that word doesn't seem to be of Greek origin. Greek words containing a upsilon were often spelled with a y in Latin, such as chrysanthemum, from Greek χρυσάνθεμον (golden flower) and lyra, from Greek λύρα. I haven't been able to find σύλβα anywhere. This dictionary says sylva is "weniger gut" (less good) than silva.