Which of the muscles on the dorsal surface are shaped like a trapezoid?

Trapezius.

Explanation:

Trapezius is a trapezium shaped muscle present on the dorsal surface of the body.Trapezius stretches from the occipetal bone at the lower back of the cranium to the lower thoracic vertebrae.Its is laterally attached to the scapula of the pectoral girdle.This arrangement gives it a typical trapezium like shape.The main functions of trapezius is to assist the movement of scapula and support the arms.It also helps in movement of Spine