Within each Subclass are Orders, Families, Genera and numerous Species. For information on how this works see Mammal Classification , for a list of the 16 orders and their 133 families, see Orders . The possession of a placenta in the females which allows the mother's system to nourish the growing young without being attacked by the mother's immune system. This applies to all the Eutheria which is why they are called placental mammals. Cows, horses, dogs, cats and people are all placental mammals.
Mammals have only two sets of teeth.
Reptiles and fish have many sets and can replace lost teeth with new ones all their lives. Mammals are heterodontic, meaning that their teeth are different shapes, except those with no teeth at all. Reptiles and fish have teeth that are all basically the same, though they can vary in size throughout the mouth. See image above. Effectively, this means that if your mouth is full of food you can still breathe, but a reptile has to breathe around its food.