VERRUCA LÆVIGATA. _G. B. Sowerby._ Genera of Recent and Fossil
Shells, Plate.
_Moveable scutum, with the lower articular ridge broader than the short
upper articular ridge; moveable tergum broader than high, with the
upper articular ridge produced into a point._
_Hab._--Tierra del Fuego; Eastern Patagonia, nineteen fathoms; Chile;
Peru; Mus. Brit., Cuming, Stutchbury, Darwin: attached to shells, and
often to _Balanus lævis_ and _psittacus_.
I can point out no difference in the shell between this species and
_V. Strömia_, excepting that its walls seem invariably to be smooth,
which is rarely the case with _V. Strömia_; perhaps also the oblique
interfolding articular plates between the several compartments are here
more prominent. It appears that specimens with the left side uppermost,
and therefore with the left opercular valves moveable, are considerably
more common than those with the right valves moveable. The moveable
scutum and tergum are articulated together by much more prominent
articular ridges than in _V. Strömia_, and the two valves together are
broader in proportion to their height,--the height being measured from
the apex to the basal margin. In the scutum the lower articular ridge
is considerably broader than the short upper ridge. In the tergum, the
basi-carinal corner is more rectangular, and the whole valve is nearly
square: owing to the deep furrow receiving the lower articular ridge of
the scutum, the axial ridge of the tergum is proportionally narrower
but more prominent than in _V. Strömia_; the uppermost ridge (formed by
the occludent margin of the valve) projects, especially when viewed on
the under side (fig. 3 _b_), as a moderately sharp point.
In the _mouth_, the lower teeth of the mandibles are more distinct than
in _V. Strömia_; the lower part of the edge of the maxilla is very
prominent. In the second and third pairs of cirri the terminal spines
on the shorter rami are coarsely pectinated; on the sixth pair there
are only three pairs of main spines on each segment; but these several
points, according to the analogy of other species, I should expect to
be variable.
This species is alluded to by Bruguière, in the 'Encyclopédie
Méthodique,' but was confounded by him with the _V. Strömia_ of Europe.