_Shell white or gray: sheath and opercular valves generally clothed by
fimbriated membrane: tergum, with its basi-carinal angle depending and
pointed._
_Hab._--Peru, Chile, Chiloe, Northern Chonos Islands. Attached to
littoral rocks, and sometimes to littoral shells, often mingled with
_Chthamalus scabrosus_; Mus. Brit., Cuming, Darwin.
_General Appearance and Structure of Shell._--Shell dirty white or
gray: sometimes tinted pale purple within; irregularly conical, or
much depressed, or cylindrical and much elongated. Generally much
corroded, sometimes well preserved and covered by membrane. Orifice
rather large, of variable shape. Sutures often quite obliterated.
Radii when developed narrow, with their sutural edges, as well as
those of the alæ, generally very finely crenated, but to a variable
degree. The membrane lining the sheath and covering the opercular
valves, is remarkable from each zone being fimbriated; for this
expression is more correct than to say that the membrane bears a row
of spines, though the fimbriæ do approach in character to spines;
sometimes, though rarely, the fimbriæ are branched. The largest
specimens which I have seen (from Coquimbo and Valparaiso) were half
an inch in basal diameter, and some of these were so much elongated
as to be one inch in height.
_Scuta._--The scuta are rather narrow: they have a somewhat peculiar
appearance, from the articular furrow being wide, and from the
articular ridge projecting with a uniform curvature: the pit for the
lateral depressor muscle has some minute crests, of which I have seen
traces in the foregoing species. The _Terga_ vary somewhat in shape:
they have the basi-carinal angle of the valve, where the narrow
crests for the depressores are placed, pointed and dependent, and
the surface of the valve above these crests is prominent. Altogether
the opercular valves have a sufficiently distinct character to be
recognised without much difficulty.
_Mouth._--The crest of the labrum is not toothed; the palpi have long
hairs along the exterior basal margin. The lower main teeth of the
mandibles are plainly laterally double. _Cirri._--The pedicel of the
second cirrus is extremely broad, and on the exterior margin supports
a tuft of very long, finely plumose spines: in some specimens each of
the lower segments of the anterior ramus of this cirrus bore one or
two very large spines, doubly and extremely coarsely pectinated. In
two specimens the rami of the third cirrus were of equal length: but
in one specimen (from Iquique, Peru), having seven segments in the
shorter ramus of the first and second pairs of cirri, the posterior
ramus of the third pair had fourteen segments, and the anterior ramus
twenty-two segments. The posterior cirri have segments carrying five
pairs of main spines: the dorsal surfaces of the lower segments are
serrated.