(sect, c), balanus concavus. 239 



small, and moderately reflexed : the adductor ridge (as already stated,) 

 varies remarkably ; in most of the Panama specimens, it is extremely 

 prominent, and extends down to near the basal margin ; in other spe- 

 cimens it is but slightly prominent, especially in some of the fossil 

 specimens from Virginia. The cavity for the lateral depressor, also, 

 varies greatly ; it is often bounded on the side towards the occludent 

 margin by a very slight straight ridge, which occasionally folds a little 

 over, making almost a tube ; this, at first, I thought an excellent 

 specific character, but far from this being the case, the cavity often 

 becomes wide, quite open, and shallow. 



Terga, very slightly beaked ; the surface towards the carinal end of 

 the valve, in some of the fossil specimens, is very slightly striated longi- 

 tudinally. There is either a slight depression, or more commonly a deep 

 longitudinal furrow, with the edges folded in and touching each other, 

 extending down the valve to the spur, and causing the latter to vary in 

 width relatively to its length. When the furrow is closed in, the spur 

 is about one fourth of the entire width of the valve, and has its lower 

 end obliquely rounded, and stands at about its own width from the 

 basi-scutal angle : when there is only a slight depression and no 

 furrow (as is always the case with young specimens), the spur is 

 broader, equalling one third of the width of the valve, with its lower 

 end almost truncated, and standing at about half its own width from 

 the basi-scutal angle. But the absolute length of the spur, also, varies 

 considerably ; it is often very long, compared to the whole valve. The 

 basal margin on the carinal side is sometimes slightly hollowed out ; 

 when the furrow is closed, this latter side slopes towards the spur. In- 

 ternally, the articular ridge and crests for the tergal depressor muscle 

 are moderately prominent. 



Parietes, the longitudinal septa sometimes stand near each other, 

 making the parietal pores small. The radii have oblique summits, 

 but to a variable degree ; their septa are unusually fine, and are denticu- 

 lated on their lower sides ; the interspaces are filled up solidly. The 

 alee have their summits very oblique, with their sutural edges nearly or 

 quite smooth. In most of the fossil specimens, and slightly in some 

 of the recent specimens, the surface of the sheath presents an unusual 

 character, in a narrow, longitudinal, slightly raised border, running along 

 the sutures, on the carinal side of each compartment. 



Basis thin, porose ; sometimes with an underlaying cancellated layer. 



Mouth : labrum with six teeth : mandibles with the fourth and fifth 

 teeth small, either sharp, or blunt : maxillae with a straight edge, or with 

 the inferior part slightly prominent. Cirri with the rami of the first 

 pair unequal by four or five segments : the segments in the shorter 

 ramus are extremely protuberant. The segments in the second cirrus 

 only moderately protuberant : but all the specimens were in bad con- 

 dition, and it appeared as if, in the Panama specimens, the segments of 

 the second cirrus were more protuberant than in the Philippine Island 

 specimens. In the posterior cirri there are from three to five pairs of 

 spines on each segment : even amongst the Panama specimens some 

 had three and some four pairs, and a white Panama specimen had five 

 pairs of spines. 



