I think Nala is a lab/mutt mix. ^_^

this site explains that not all dogs are the decendents of purebreds, and it's nearly impossible to figure out what is in a mutt.

And as cali said, there are two types of labs, the leaner ones, and the stockier ones. Nala looks like she is from the leaner labbie line.

Whatever she is, she is adorable, and that's all that matters.