I double checked again, since I've been 'round and 'round on the CON thing (I actually didn't believe it when I first checked).
Wildshape is same as polymorph except as noted.
Polymorph is the same as alter self except as noted.
The line you're speaking of appears here, quoted from the SRD. From the Alter Self spell:
However, from Polymorph:
If you gain the new constitution score, your hit points change. Thus polymorph is notated differently than Alter Self in this respect -- polymorph takes precidence according to wildshape.
Be a warrior.
edit: Sorry, just noticed your group ruled that HP doesn't change -- well, there's the argument if you want to beg.
Incidentally, you just take the CON modifier of the new creature and multiply the difference in modifiers between your current CON and your new CON by your level.
So if you have CON 10, the critter you want to switch into has CON 14, it's then (2 x [your level] = HP).
Don't wade into any anti-magic fields while you're shaped and hurt though -- you may be surpressed just long enough to die immediately.
Wildshape is same as polymorph except as noted.
Polymorph is the same as alter self except as noted.
The line you're speaking of appears here, quoted from the SRD. From the Alter Self spell:
You retain your own ability scores. Your class and level, hit points, alignment, base attack bonus, and base save bonuses all remain the same.
However, from Polymorph:
The subject gains the Strength, Dexterity, and Constitution scores of the new form but retains its own Intelligence, Wisdom, and Charisma scores. It also gains all extraordinary special attacks possessed by the form but does not gain the extraordinary special qualities possessed by the new form or any supernatural or spell-like abilities.
If you gain the new constitution score, your hit points change. Thus polymorph is notated differently than Alter Self in this respect -- polymorph takes precidence according to wildshape.
Be a warrior.

edit: Sorry, just noticed your group ruled that HP doesn't change -- well, there's the argument if you want to beg.

So if you have CON 10, the critter you want to switch into has CON 14, it's then (2 x [your level] = HP).
Don't wade into any anti-magic fields while you're shaped and hurt though -- you may be surpressed just long enough to die immediately.

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