Why is the relation in 3NF?

> A relation is in 3NF (Third Normal Form) if it is in 2NF (Second Normal Form) and all the FDs (functional dependencies) in the relation are in the form X -> Y, where X is a superkey (a set of attributes that uniquely identifies a tuple in the relation) and Y is a non-prime attribute (an attribute that is not a part of the primary key).

SP (Sno, Sname, Pno, Qty)

Functional dependencies:

Sno, Pno -> Qty
Sname, Pno -> Qty
Sno -> Sname
Sname -> Sno

The candidate keys are Sno, Pno and Sname, Pno. So the first 2 are in the form quoted above while the last two aren't. For instance in the case of Sno -> Sname, Sno is not a superkey and Sname is a prime attribute. So why is the relation in 3NF?