Complex sentence: In primitive placental mammals there are four premolars per quadrant, but the most mesial two (closer to the front of the mount) have been lost in catarrhines (Old World monkeys and apes, including humans).
Split into simpler sentences: In primitive placental mammals there are four premolars per quadrant. The most mesial two have been lost in catarrhines (Old World monkeys and apes, including humans).