They might also be crimini which are also sometimes called baby bellas. From an article by Faith Durant:
Did you know that most of the table mushrooms we eat are all of the same variety? Its name is Agaricus bisporus, according to Wikipedia, and it includes portobello, cremini, and white button mushrooms.
• The difference between these popular varieties of mushrooms is just age. The white button mushrooms, those very familiar kitchen staples, are simply the youngest variety. They have been cultivated, too, for that white color and soft texture. In the wild these mushrooms are usually browner.
• The portobello is the most mature mushroom here; it’s really just an overgrown white mushroom! They are left to grow for longer, until they have spread out into that delicious meaty cap.
• The cremini mushroom, then, is just in between these two varieties. It’s a moderately mature version of the white button mushroom, which is why it has a similar flavor. It’s younger than the portobello, but still related, which is why these are sometimes sold as “baby bella†or “baby portobello†mushrooms.