66 FOSSIL CIRRIPEDIA. 
form the variation above alluded to. But, after careful comparison of a large series, I feel 
convinced that the difference is caused solely by age: the chief difference consists in the 
occludent margin being considerably longer than the scutal margin, and consequently 
in the widest part of the valve lying below the middle pomt: that portion of the valve, 
moreover, which lies on the carial side of the sub-central ridge is, in proportion, narrower 
than in the common form. Internally, in these large specimens, a considerable portion of 
the apex is marked by lines of growth: it follows from this, that if only the imternal 
corium-covered surfaces of the large and small terga be compared, there is far less 
difference of outline than if they be compared externally. I repeat that I have no doubt 
that these valves all belonged to the same species. 
Carina (Tab. 4, fig. 1a), closely like that of P. glaber, triangular, moderately tapering, 
very slightly bowed inwards; smooth, sub-carinated, transversely slightly convex; lateral 
margins very narrowly inflected, with the lines of growth curved downwards, and those on 
the edge itself upturned; there is in this specimen a narrow, slight, linear channel along 
the line where the lines of growth are upturned; a small portion of the apex must have 
projected freely ; basal margin (and consequently lines of growth) rounded, less angularly 
protuberant than in P. glaber, with the two baso-lateral angles slightly prominent. 
Rostrum (fig. le), triangular, rather more than half as wide and about half as long as 
the carina, therefore rather wider in proportion to its length; more bowed inwards; a very 
small portion of the apex, which is internally simply concave, projected freely ; basal margin 
curved, slightly protuberant: exterior surface smooth, sub-carinated, like the carina; edges 
very thin. Swd-rostrum (f) about half as long, and two thirds as wide as the rostrum ; 
apex rounded ; basal margin not protuberant ; internally, slightly concave, with thin lateral 
margins, widely overlapping the latera of the lower whorl; exterior surface not sub- 
carinated, that is, destitute of a central ridge. 
Upper Latera (g), elongated, triangular, flat, exteriorly smooth, except from the lines 
of growth; two upper sides almost exactly equal in length, and equalling once and a half 
of the length of the basal margin; entire valve rather exceeding half the length of the 
terga. 
Lower Latera: these consist of two small valves (/, 4), namely (judging from the 
position in which, overlapping each other, they were embedded), the first and second, or 
more probably the second and third right-hand rostral latera of the lower whorl; and a 
pair (4, 2) (right-hand and left-hand) of latera, of about twice the size of the two anterior 
ones, which must have come from the carinal half of the whorl, but the exact position of 
which I cannot tell. These latter larger latera are thin, and considerably convex ; they are 
transversely elongated, and, in their longer axis, are rather more than half the length of the 
terga of the same individual. In shape they may be almost compared to one valve of a 
Donax ; being sub-trigonal, with the two upper sides unequal in length, and with the third 
side arched and protuberant at a point about one third of the entire length of the valve 
from the narrower end: to this protuberant point, a well-defined ridge runs from the apex 
