That's because they share the defining characteristics of mammalia and are evolutionarily linked to the rest of the class.

这是因为它们拥有哺乳动物的界定性征，而且在进化上与其他哺乳动物有 千丝万缕的联系。

They may be lizards or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals.

它们可能是蜥蜴或小鸟， 或非常年轻的哺乳动物， 这些动物的头部和躯干的形成方式非常相似。