20 FOSSIL CIRRIPEDIA. 
and in the fossils from Portugal, it is extremely prominent, and extends down to near the basal 
margin; in other specimens it is but slightly prominent, as in those from the Crag (4/) ; 
it is short, but rather prominent in the specimens (44) from Maryland; whereas it is very 
slightly prominent in the specimens from Virginia. ‘he cavity for the lateral depressor, 
also, varies greatly ; it is often, as in the recent specimens, 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, in recent specimens as well as in the crag 
specimens (4/), wide, quite open, and shallow. The whole valve in the Crag specimens 
(fig. 4e) is apt to be more elongated than in the recent or Portuguese specimens (fig. 4), 
and especially than in the Maryland (fig. 44) specimens. 
Terga very slightly beaked; the surface towards the carinal end of the valve, in 
some of the fossil specimens, is feebly striated longitudinally. There is either a slight 
depression (fig. 44), or more commonly a deep longitudinal furrow (fig. 4g, 40) 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, 
and in the specimens (44) from Maryland), 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 consider- 
ably in the Coralline Crag specimens ; it is often very long, (fig. 4y) compared to the whole 
valve. In many Italian specimens (40) it is long and broad. ‘The basal margin of the 
valve on the carinal side of the spur is sometimes slightly hollowed out; and when the longi- 
tudinal furrow is closed, this side slopes considerably towards the spur. Internally, the 
articular ridge and the crests for the tergal depressor muscles are moderately prominent. 
Parietes, the longitudinal septa sometimes stand near each other, making the parietal 
pores smal]. The radi have oblique summits, but to a variable degree; their septa are 
unusually fine, and are denticulated on their lower sides; the interspaces are filled up 
solidly. The ale have their summits very oblique, with their sutural edges nearly or 
quite smooth. In most of the fossil specimens (‘T'ab. I, fig. 44, 7), and slightly in some of the 
recent specimens, the surface of the sheath presents an unusual character, in a narrow, lon- 
gitudinal, slightly raised border, running along the sutures, on the rostral side of each suture. 
Basis thin, porose ; sometimes with an underlaying cancellated layer. 
All the recent specimens which I have seen, were, with one exception, attached to 
various shells and crabs, and to each other. ‘Phe tertiaxy specimens are often congregated 
together into great masses. Including the recent and fossil specimens, this species encircles 
the globe. During the miocene period it seenis to have been the commonest existing 
Sessile Cirripede ; now, it does not appear to be common, excepting, perhaps, at Panama. 
