By about 40 million years ago, there was the Dorudontinae. They were similar to dolphins, and grew to length of 16 ft or so. Their nostrils were still on the snout, but further back than Protocetidae, and their bodies more streamlined.
Between 24 and 34 million years ago, the two groups Odontoceti and Mysticeti arose. Among the many primitive Odontoceti, the Squalodontae was closest to modern dolphins, and is probablythe group from which most later odontocetes ultimately derived. These ceteceans were 10 ft or more in length, but still retained one primitive feature: that of heterodont dentition (ie: they had more than one kind of tooth. Modern dolphins have homodont dentition, meaning all their teeth are essentially alike.)
About 24 million years ago, a relatively diverse family known as Kentriodontidae appeared in both the Atlantic and Pacific oceans. They were about 6 feet or less in length. It is believed that the modern superfamily Dephinoidea (which includes almost all toothed whales, including dolphins), arose from this family about 10 million years ago.