Complex sentence: The deer was of species "Cervus elaphus", which has been described as "the noblest of (our) indigenous species", and the largest extant species in the British Isles.
Split into simpler sentences: The deer was a "Cervus elaphus", which has been described as "the noblest of (our) indegenous species". It is the largest extant species in the British Isles.