There is no hard and fast rule but in general the surname is usually given for the head of household. Subsequent entries with the same surname are left blank or indicated by ditto marks. When a new surname occurs it is written in.
As no family relationships are shown in 1841, one has to make a few assumptions.
Isabella, 45, was a Mearns, presumably the wife.
Mary, 18, Ritchie - ? sister to Isabella 20