A, rostrum.
B, carina.
S, moveable scutum, S′, scutum fixed and modified so as to
form part of the shell.
T, moveable tergum, T′, tergum fixed and modified so as to
form part of the shell.
In S, and S′, _a_ is the occludent margin; _b_, the basal
margin; _m_, the plate to which the adductor muscle is
fixed; _o_, a ledge formed during continued growth, in
the upper part of the fixed scutum, in order to keep the
orifice perfectly closed; this ledge is seen distinctly
only in fig. 5.
In S and S′ the tergal margin is marked by small dashes; (′)
being the upper articular ridge, and (′′), the second or
lower articular ridge: in S′ (′′) is called the parietal
portion of the valve.
In T and T′, the scutal margin is marked by small dashes; (′)
being the first and upper articular ridge, hardly distinct
from the occludent margin, and called in T′′ the occludent
rim; (′′) is the second or middle, and (′′′) the lower or
third articular ridge, called in T′′ the parietal portion
of the valve; _x_ is the carinal margin, called in T′ the
carinal rim, and _z_ the basal margin; _o_ is the occludent
margin: in T′, a ledge is formed across the under side
close to the apex, which ledge is necessary to keep the
orifice neatly closed, owing to the apex of the moveable
tergum coming to project freely during growth.
Fig. 1 _a_, Verruca Strömia, specimen with the left hand scutum
and tergum fixed and modified into part of the shell.
1 _b_, Verruca Strömia, (with left hand scutum and tergum
fixed), with the valves and compartments separated from
each other; the homologous parts of the opercular valves
are marked by corresponding letters and dashes.
1 _c_, Verruca Strömia, left hand scutum and tergum fixed,
viewed from the under side.
1 _d_, Verruca Strömia, _smooth var._, reversed specimen, _i.
e._, with the right hand scutum and tergum fixed and
modified into part of the shell.
1 _e_, Verruca Strömia, shell seen from the under side; the
right hand scutum and tergum (as in 1 _d_) being the fixed
pair: the moveable scutum and tergum (S, T) are seen in the
shade at the bottom of the shell, and their shape will be
best understood by looking at fig. 1 _f_. The fixed scutum
and tergum S′, T′, differ a little in shape, and in the
form of their line of junction, from the same two valves
(reversed) in fig. 1 _c_.
1 _f_, Verruca Strömia, moveable scutum and tergum, seen from
the under side, taken from a specimen, in which (as in 1
_d_ and _e_) the right hand scutum and tergum were the
fixed pair.
2, Verruca Spengleri, scutum seen from the under side, showing
the medial, prominent adductor ridge; taken from a
specimen, in which the right hand scutum and tergum were
fixed.
3 _a_, Verruca lævigata, scutum and tergum, external view of,
from a specimen in which the left hand pair was fixed; 3
_b_, scutum and tergum, internal view of, from a specimen,
in which the right hand pair was fixed.
4, Verruca prisca, scutum and tergum, external view of, from a
specimen, in which left hand pair was fixed.
5, Verruca nexa, with the valves separated, the left hand
scutum and tergum being the fixed and modified pair.
6, Portion of ribbed shell of a Venus, to which a _Verruca
Spengleri_ had been attached, showing the peculiar form of
the excavation.