The mammals are animals that are born alive and fed with milk from the
mother’s breast. They are warm-blooded, air-breathing, have a protective
covering of hair, and are the most advanced of all vertebrates. The
foregoing features are the more typical mammalian characteristics, but
exceptions to these are found in certain mammals.
Mammals first appeared in the Jurassic and were probably derived from
some form of mammal-like reptile. Although rare during the Mesozoic,
mammals underwent rapid development and expansion during the Cenozoic,
and during this era certain types of mammals became extremely large and
assumed many bizarre shapes. The majority of these unusual forms lived
but a short time but are well known from their fossils, and the remains
of some of these animals which inhabited Texas during the Cenozoic may
be seen in the Texas Memorial Museum at Austin.
Recent mammalian classification recognizes several subclasses and
numerous orders and suborders, but the treatment of the mammals in a
publication of this nature must of necessity be somewhat brief and no
attempt at detailed classification is made.