| The largest uppermost flat bone of the gill-covers is known as the opercle. |
| The spinous armature of the opercle and the two spines of the first dorsal fin constitute the weapons. |
| On our sunfish we see that the opercle ends behind in a long and narrow flap, which looks like an ear. |
| In most of the higher fishes a small accessory gill is developed in the skin of the inner side of the opercle. |
| In Erpetoichthys only the opercle is distinct among the gill-covers. |
| The tip of its opercle is prolonged like an ear and jet black in color, with a dash of bright scarlet along its lower edge. |
