Note 23 — 1. Family BALANIDÆ, (or Sessile Cirripedes).

In some species of Pyrgoma, the ledge (limbus occludens) which is added along the occludent margin of both scuta and terga, and in some species of Balanus a narrow rim, or slight protuberance which is added along the carinal margin of the terga, offer unimportant exceptions to the rule, that the opercular valves grow only at their basal margins.