Simple Q&A D&D 3.5 (by RAW) XXVII

[Oddman80]

A 67

Not sure what accompanying chart you mean. The animal gains extra hit points equal to its total CON modifier for each HD it possesses. For example a Wolf companion (CON 15, +2 modifier) of a 3rd level druid has 2¹+2² HD and thus has 2d8+2*2³ + 2d8+2*2³ hit points.

¹ normal for a wolf
² Bonus HD for being a 3rd level druid
³ For CON 15

That was a complicated explanation.... though it did make me realize I was TOTALLY misunderstanding what the text was saying.
I had been thrown off by the "gains a constitution modifier." part of the text. I was trying to figure out how you add to the modifier itself (facepalm). What they meant was simply add a con modifier's value of hit points on to each hit die the companion gains... I'm an idiot.