Board Thread:General Discussion/@comment-186.36.192.93-20131208055743/@comment-8203434-20131216003944

There are no creatures, AFAIK, that have both gills and lungs so the designation for Niaids on that basis would be wrong. In any case the word "Mammal" comes from the same root as "Mammary" and refers to the class of warm-blooded creatures with an endoskeleton who give birth to live young. I suspect that there is a requirement for the female to produce milk in the definition somewhere, either extant or in a prior version.