[22] In comparing the Tergum of one of the Balanidæ with that of a typical member of the Lepadidæ, for instance, that of Balanus with that of Pollicipes, apex corresponds with apex: the extremity of the spur in Balanus corresponds with the basal point of the whole valve in Pollicipes: the scutal margin, (which in Balanus homologically extends down to the extremity of the spur), corresponds with the scutal margin of Pollicipes: the carinal margin in Balanus corresponds with the _upper_ carinal margin in Pollicipes: the basal margin of Balanus on the carinal side of the spur, corresponds with the _lower_ carinal margin in Pollicipes: lastly, (and this is the chief difference), in Balanus there is no appreciable occludent margin, the apex of the valve being brought close to the upper angle of the scutal margin; in Chthamalus, however, there is yet left some remnant of an occludent margin,--which margin in Pollicipes is conspicuous. The scutum and tergum, with the few exceptions above stated, are articulated together at a large or open angle. The articulation (see Pl. 11, fig. 5 _b_, _c_, _d_, and fig. 6 _b_, _c_) is effected by the margin of the tergum being a little inflected, and lodged in a furrow in the margin of the scutum. This furrow in the scutum has its further border generally prominent and often reflexed or curved over; I have called it the articular ridge; it, also, is lodged in a furrow in the upper part of the tergum, which again is bordered by a ridge, viz., the articular tergal ridge. So that in both scutum and tergum there is an articular furrow, bordered in each case, on one side by the margin of the valve, and on the other side by the so-called articular ridge. In Chelonobia (Pl. 14, fig. 1 _b_) the articular ridge of the scutum is horny. When, as often happens, the scuta and terga have been much worn, the manner of their articulation (Pl. 18, fig. 1 _a_) is pretty well shown even from the outside; in this case their external appearance is very different from what it is in those individuals (fig. 1 _c_) of the same species, which have not suffered disintegration. This articulation of the scuta and terga is prefigured amongst the Lepadidæ, in _Pollicipes mitella_, and in Lithotrya.