_Shell, apparently, as in A. spongites, but larger: scutum marked by
longitudinal ridges, often in pairs, with the intermediate furrows
rather wide: spur of tergum nearly half as wide as valve._
_Fossil_ in Coralline Crag (Sutton), Mus. S. Wood.
I owe to Mr. Wood the inspection of a fine suite of separate valves.
Owing to the shell never having been found entire, its general shape is
not known, and, what is of more consequence, the relative proportional
width of the parietes of the carino-lateral compartment is unknown. I
have (but with doubt) given it a distinct specific name, owing to the
peculiar character of the furrows on the scuta, and to the large size
of the whole shell. In its other characters it comes nearest to _A.
spongites_, excepting in the spur of the tergum, which resembles that
of _A. sulcata_.
The compartments appear to have been rather smooth externally. The
radii are not wide, as in _A. cyathus_; and the basis is cup-formed.
Internally, the parietes are feebly ribbed, as in _A. spongites_.
Judging from the dimensions of the separated valves, this species
must have equalled and perhaps exceeded the size of the largest
living species, namely, _A. glans_, from Australia. Hence we may
infer, that the basal diameter probably exceeded .55 of an inch: I
may add, that the largest European specimens of _A. spongites_, from
Naples and Portugal, are only .3 of an inch in basal diameter.
_Scuta._--These seem to resemble the scuta of _A. spongites_ in all
respects, except in the longitudinal ridges standing much further
apart, and, consequently, in the furrows being much wider: each ridge
is generally double. Although there is a good deal of variability
in the character of these ridges in _A. undulata_, and likewise in
_A. spongites_, I have not seen any form intermediate between them.
It must, however, be confessed, that this is an extremely variable
character in many sessile cirripedes. In the _Terga_, the spur is
about half the width of the whole valve, and therefore rather wider
than in _A. spongites_.