The whole of the "Devonshire-Ellis" is the person's family name. In Britain such hyphenated names, sometimes called 'double-barrelled' names, usually come about because a couple decide that their child should inherit not just the father's name but also the mother's. Occasionally they result from a name change in later life, for example, a man might on marriage hyphenate his wife's name with his own family name.
In the British aristocracy childless wealthy landowners might leave their money to someone on condition that they change their name or take an additional hyphenated family name.
Hyphenated names at one time held a certain social cachet - the suggestion that the person might have grand aristocratic connections. In more recent times, however, a hyphenated name is sometimes given to a child when the parents, of whatever social class, are not married, or, if they are, if the mother has not adopted her husband's family name.
There is no limit on the number of names that can hyphenated together: for example, the full family name of the actor Ralph Fiennes is Twisleton-Wykeham-Fiennes, and Admiral the Hon. Sir Reginald Aylmer Ranfurly Plunkett-Ernle-Erle-Drax, was a British naval officer (his case is also an example of hyphenation being acquired in adult life).