(sect, c), balanus amphitrite. 245 



valve is narrower, the spur is much sharper arid narrower, the carinal 

 half of the basal margin is much hollowed out and slopes down 

 towards the spur, with the crests for the depressor muscles depending 

 beneath the basal margin, and with the carinal margin sometimes 

 extremely convex or protuberant. But the shape and position of the 

 spur, and the outline of the carinal half of the basal margin vary much 

 in nearly all the varieties. 



Compartments. — The upper parts of the parietal pores are either 

 filled up solidly with, generally coloured, shell, or they are crossed by thin 

 transverse calcerous septa : the longitudinal parietal septa occasionally 

 bifurcate at their bases close to the outer lamina, making an irregular 

 outer row of minute pores. The Radii have their septa rather fine, 

 and finely denticulated on both sides, but sometimes only on the lower 

 side ; the thickness of the septa varies a little ; the interspaces are 

 filled up solidly ; the summits of the radii are jagged and oblique, and 

 usually form an angle of about 45° with the basis, not being added to 

 above the level of the opercular membrane ; but not rarely they reach 

 up much higher, and are very nearly parallel to the basis, extending 

 from tip to tip of the compartments. Again, in some ordinary varieties, 

 and always in var. Stutsburi, the summits of the radii are extremely 

 oblique, the radii themselves forming a mere border to the compart- 

 ments to which they belong. In no other species have I seen so great 

 an amount of variation in the form of the summits of the radii. The 

 alee, in like manner, have their summits either very oblique, not being 

 added to above the opercular membrane, or they are only slightly 

 oblique ; it often happens that in those specimens in which the sum- 

 mits of the radii are nearly parallel to the basis, the ahe are very 

 oblique, and the converse : in other individuals, both radii and alae have 

 equally oblique summits. The sutural edges of the alss vary in thick- 

 ness, being either very thin and obscurely crenated, or moderately 

 thick and ribbed. The basis is porose ; but I have never seen an 

 underlying cancellated layer of shell, as is so common in several 

 species. 



Mouth : labrum, with from four to eight, generally with six, little 

 teeth : mandibles with three teeth, and two minute lower teeth, or 

 mere knobs: maxillae with the edge straight, or with the inferior part 

 forming a slightly step-formed projection. Cirri: the rami of the 

 first pair are unequal by three or four segments, but in some specimens 

 by five or six segments, with the front surfaces of the segments in the 

 shorter ramus extremely protuberant. The second pair of cirri are 

 short, with the front surfaces of the segments moderately protuberant : 

 the third pair have a tuft of bristles at their bases on the thorax. The 

 segments in the sixth pair have from four to six pairs of spines on the 

 segments ; equal-sized specimens seem to vary in this latter respect. 

 There is a small sharp projection on the dorsal base of the penis. 



Varieties. 



With respect to var. 1, communis, I have nothing further to remark, 

 except that I have seen specimens identically similar from the Medi- 



