egular width,
bordered on each side by a very minute ridge, which is distinctly
continuous with the inner edge of the occludent margin, both above and
below the cavity. The furrow appears to have been formed by calcareous
matter not having been deposited along this line, during the thickening
or growth of the internal surface of the valve: I suspect, that it
originates at a single period of growth, for I could see no signs of
successively-formed transverse lines. I believe that it is strictly
homologous with the fold, over which the complemental male is attached
in _S. vulgare_, but carried, for a special purpose, much further across
the valve and rectangularly inwards, for in structure and position both
are identical. In comparing the internal views of the scuta in _S.
vulgare_ and _S. ornatum_ (Pl. V, fig. 15 _a'_, and Pl. VI, fig. 1
_c'_), it must be borne in mind, that the latter should be compared, as
clearly shown by the lines of growth, with that portion alone of the
scutum in _S. vulgare_, which lies under the curved ridge connecting the
umbo and tergo-lateral angle. The deep cavity in which the male is
lodged, is formed subsequently to the preparatory furrow, simply by the
gradual thickening of the surrounding surface of the valve, more
especially of a ridge just above the pit for the adductor muscle, and of
another broad ridge just beneath the tergal margin. The deepest part of
the cavity lies parallel to the tergal margin along the upper side, and
here, in the older valves, the preparatory furrow can by care be
distinctly traced. In conformity with the shape of the cavity, the
orifice or notch in the occludent margin of the scutum, is situated at
the point where the preparatory furrow sweeps round and enters. I
believe that the cavity is lined by membrane, and that between the
cavity and the body of the female, there is a complex membranous
layer,--a pouch or bag being thus formed. An imaginary section of this
pouch (with the thickness of all the parts extremely exaggerated and in
a reversed position) is given in Pl. VI, fig. 1 _d'_: _a_ is the shell;
_x_ the cavity, converted, as I believe, into a pouch by, firstly, the
delicate tunic (_c_) lining the sack of the female; secondly, a double
layer (_d_) of corium; and, thirdly, by a special, rather thick
membranous layer (_b_), which thinning out round the cavity coats only
part of the under surface of the scutum. This latter membrane I have not
seen in any oth