Basically, there are two different types of cats– ones that can purr and ones that can roar. These groups are divided based on the structure of a series of connected bones at the base of the tongue, called the hyiod apparatus. In one group of cats, one of the hyoid bones fail to develop and they have only a threadlike ligament, so that tongue and larynx are loosely attached to the base of the skull. This group includes the big cats: the tiger, lion, jaguar, leopard, and ounce (snow leopard), which are classified in the genus Panthera or Leo; the clouded leopard also is sometimes placed in this genus. These cats are unable to purr. Their voice is a roar and the pupils of their eyes are round. In the second group, the hyoid develops normally. All such cats purr, but do not roar. The pupil is usually vertical but may be round in a few species. This genus, Felis includes the mountain lion and numerous kinds of smaller cats including the domestic cat. The Canada lynx and bobcat are often placed in a separate genus, Lynx. The cheetah, or hunting leopard, has a normal hyoid but has nonretractile claws and differs from all other cats in other ways; it is placed alone in the genus Acinonyx.