Females additionally have a mammary slit on either side of the genital slit, enclosing the nipples. In the forehead of all species is a bulge called the melon. Dolphins have an astonishing sonar capability. They send out a series of clicks, so short and close together that they sound like a single continuous noise to us, although they are usually outside our range of hearing. The echoes that bounce off the dolphin's surroundings give the dolphin a 3-D picture in remarkable detail.
Dolphins have good eyesight, too, but sonar obviously has some advantages over vision.
First, it can be used in the dark just as well as by day. It can be used in cloudy or murky water as easily as clear water. For example, one day, a dolphin seemed particularly excited and swam around a woman who worked at a water park, aiming sonar clicks at her abdomen. A doctor visit a week later confirmed the woman's pregnancy. I think it is impossible, since the sonar clicks are created using air trapped within cavities near the trachea.