THE SESSILE BARNACLES. 179 



The barnacle is conic or cylindric-conic, rarely elongated, colum- 

 nar ; dirty white, with the surface generally ribbed or plicate closely 

 and deeply, without epidermis, and often eroded. The orifice is 

 typically small, but is large in some lots. Usual diameter 14 mm., 

 height 7 to 8 mm.; often smaller. 



The scutum has fine growth-ridges on the flat exterior. The articu- 

 lar ridge is very prominent, articular furrow wide and deep. The 

 adductor ridge is very short, united above with the articular ridge, a 

 pit ~below their junction. Adductor pit deep. Pit for the depressor 

 muscle small, marginal, and usually deep. The upper part of the 

 interior is not striate. 



The tergum is flat or has a slight depression leading to the spur. 

 The articular ridge and inflexed scutal margin are very prominent, 

 the articular furrow deep. Spur short, occupying nearly half of the 

 lower margin, truncate, and standing very close to the basiscutal 

 angle. Crests for the depressor muscles are conspicuous. The lining 

 membrane of the valves is black. 



The radii are narrow or very narrow, not conspicuously differen- 

 tiated from the parietes. Their summits are very oblique and smooth. 

 The articulating edges are broad and very finely laminate. The 

 opposed sutural edges also have a broad, minutely laminate band. 

 The alse are rather wide with the edges simple. Sheath pale yellow, 

 with transverse raised lines. Below its depending edge there is a 

 rather broad, shallow excavation. The inside of the parietes is 

 smooth above in adults, becoming very strongly ribbed near the 

 base, where the ribs are conspicuously crenulated. The pores in 

 immature individuals are small and irregular, and in most adults 

 they are nearly or entirely wanting, being filled up with calcareous 

 material; but sometimes traces of them may be seen at the worn 

 summits of the parietes. 



The basis is flat, thin, and without pores, its inner face finely 

 grooved radially. 



It was described by Darwin, as follows: 



Mouth : Labrum with the central notch rather widely open, with four teeth on 

 each side of it. Palpi with very short spines along their inner margins. Man- 

 dibles with the fourth and fifth teeth forming mere knobs. Maxillse small, with 

 a mere trace of a notch under the two great upper spines. Cirri : First pair with 

 the rami unequal by three or four segments, the longer ramus being only one- 

 quarter of its own length longer than the other ramus. Second pair short, with 

 the segments (and those of the shorter ramus of first pair) somewhat pro- 

 tuberant. Third pair with the rami one-third longer than those of the second 

 pair. Sixth pair with the upper segments elongated, and bearing six or seven 

 pairs of spines. 



The cirri are densely pigmented in some of the specimens I have 

 examined. The third pair have no short spinules on the segments, 



