Question

If it is not a lion. then it is a cat

Answer 1

Logically, this implication can be represented as
$$
\sim L\rightarrow C
$$
Where L = Lion and C = Cat
This is equivalent to
$$
\sim C\rightarrow L
$$
Which means if it is not a cat then it is a lion.
Finally, we can also say (see - http://en.wikipedia.org/wiki/Material_conditional )
$$
L\vee C
$$
All of this is equivalent to saying is a Lion or a Cat, that simple.