Claire, I thought you worked in one of those medical sorts of job.
Somebody loaned me a book; no pictures but lots of words (English and Latin mostly)
It went on in great detail about one method of classifying animals (by reproductive patterns)
It seems that of the large air breathers the following is the case:
Amphibians spawn eggs in water.
Reptiles and birds lay eggs on dry land
Marsupial mammals give birth to live young that then migrate to and are nurtured within a pouch.
Placental mammals give birth to and suckle live young.
It was a really interesting sort of classification.