phagus somewhat curled.

_Filamentary Appendages_, none.

_Ovaria_ filling up the peduncle and surrounding the sack, but not
extending up to the bases of the scuta and terga; I saw the ova only in
_L. truncata_; they were here oval and large, being nearly 9/400ths of
an inch in length.

_Penis_, elongated; vesiculae seminales extending into the prosoma. I
noticed the ovigerous fraena only in _L. truncata_; here they were large,
with an almost bilobed outline; the margin and whole lateral surface
being covered with elongated cylinders, finely pointed, but not enlarged
at their extremities, as are the glands observed in most of the other
genera.

_Colours._--The posterior thoracic segments, the pedicels, the anterior
and dorsal surfaces of the segments of the cirri, the caudal appendages,
and the outer sides of the trophi are, in most of the species, more or
less mottled with dark purple; parts of the interior surfaces of the
valves in some of the species are coloured fine purple.

_Geographical Distribution._--The species are found all round the world
in the tropical seas; this fact may have some connection with the
presence of soft coral-reef limestone and of massive corals in these
seas. The presence, however, of _L. cauta_ on the shores of New South
Wales, shows that the genus is not strictly tropical.

_Affinities._--Lithotrya is a well-pronounced distinct genus; although
there is a considerable difference in the shape of the valves between
_L. dorsalis_ and _L. Valentiana_, at the opposite extremes of the
genus, the strict uniformity of the internal characters shows that there
are no grounds whatever for any generic separation; moreover, _L.
Rhodiopus_ neatly blends together these extreme forms. Indeed it is not
easy to imagine a better marked series of transitional forms, than those
presented by the terga, in passing from _L. dorsalis_ through _L.
Nicobarica_, _L. Rhodiopus_, and _L. truncata_, to _L. Valentiana_.
Lithotrya has most affinity to _Scalpellum villosum_ or to _Pollicipes
spinosus_ and _P. sertus_; though the affinity is far from close. In
these two species of Pollicipes, we have seen that large irregular
calcified spines are formed at the base of the peduncle, whereas in the
other Pedunculata the scales or spines are formed exclusively round the
upper margin of the peduncle. Lithotrya, as has been remarked by Sowerby
and other authors, exhibits some affinity to the sessile Cirripedes, as
shown b