Let us assume that we have a serpentine eidolon with limbs (arms) and claws. Here are some related questions:
1) The eidolon has a bite attack by default. Can you add a gore attack without adding a second head?
2) The eidolon has a tail slap attack by default. Can you add a sting attack without adding a second tail?
3) Can you add a slam or pincers attack without adding more limbs?
4) If the answer to any of these question is "Yes", can you use both attacks from the same body part or just one or the other?
Prior to the publication of Ultimate Magic, I would have assumed that the answer to questions 1 and 4 is yes and that the answer to questions 2 and 3 is no. Now I am not so sure, since an answer of "Yes" to question 1 would seem to make the head evolution pointless.
What do the rest of you think?
And has there ever been any official word on these questions?