Archaeopteryx (n.)
definition: A fossil bird
Of the Jurassic period remarkable for having a long tapering tail of many vertebrae with feathers along each side and jaws armed with teeth with other reptilian characteristics.
Archaeopteryx is a 13 letter word that begins with an A and ends in an X

Cephalothorax (n.)
definition: The anterior portion of any one of the Arachnida and higher Crustacea
Consisting of the united head and thorax.
Cephalothorax is a 13 letter word that begins with a C and ends in an X

Haematothorax (n.)
definition: Same as Hemothorax.

Haematothorax is a 13 letter word that begins with a H and ends in an X

Impropriatrix (n.)
definition: A female impropriator.

Impropriatrix is a 13 letter word that begins with an I and ends in an X

 1