Sometimes writers leave words out. They expect their readers to know which words are left out.
The following sentence has a word that is left out in the second phrase:
A robin is a kind of bird, and a cat a kind of mammal.
Do you see the word that is left out in the second phrase? The writer did that on purpose.
Here is the same compound sentence but with the word that was left out:
A robin is a kind of bird, and a cat is a kind of mammal.
That is super easy, don't you think?