Snakes are often said to lack ears, but they do have the innermost components of the ear within their skulls and these structures function to pick up sound and allow the snake to hear. However, snakes lack both an external ear and an ear drum.
Rather than an auricle, a projecting piece of cartilage on each side of the head, most reptiles have an external tympanic membrane. In humans, this structure lies within the head and is called the ear drum. In lizards, the tympanic membrane lies flush or almost flush with the skin on the animal's head. When sound strikes the tympanic membrane, the vibrations are transmitted to the inner structures of the ear and sound is perceived.
Since snakes lack a tympanic membrane, they pick up vibrations using the vibration of bones in their skulls. They are best adapted to do so via the jawbones as they travel along the ground, but they are able to hear using their entire skull as a tympanic membrane.