The frontalis muscle (frontal belly) is muscle which covers parts of the skull. Some sources consider the frontalis muscle to a distinct muscle. However, Terminologia Anatomica currently classifies it as part of the occipitofrontalis muscle along with the occipitalis muscle.ome sources consider the frontalis muscle to a distinct muscle. However, Terminologia Anatomica currently classifies it as part of the occipitofrontalis muscle along with the occipitalis muscle.