Outer Maxillæ.—The inner margin is slightly concave, and in L. truncata alone, the bristles are hardly continuous, being interrupted in the middle part. The olfactory orifices are only very slightly prominent. The spines on all the trophi are more or less doubly serrated.

Cirri.—The three posterior pair are elongated, with their anterior surfaces not at all protuberant. The segments bear from three to five pair of spines, with a row of three or four small intermediate spines; there are, as usual, some little lateral upper rim spines; the dorsal tufts contain some thick and thin spines mingled. First cirrus is short, and placed not quite close to the second pair; the basal segments are broad and thickly paved with bristles. The second pair is rather short compared with the third pair; a varying number of the basal segments in both rami of both these cirri are protuberant, and are thickly paved with bristles; such segments are more numerous and are broader on the anterior rami than on the posterior rami. In L. cauta alone, none of the basal segments in the posterior rami of the second and third cirri are thickly paved with bristles. The pedicels of the first three pair are irregularly covered with spines; those of the three posterior pair have the spines arranged in a regular double line. Most of the spines are doubly serrated.

Caudal Appendages ([Pl. X], [fig. 23 and 24]), multiarticulate, with thin elongated segments fringed with short spines; in length generally exceeding the pedicel of the sixth cirrus, and in L. Nicobarica equalling half the entire length of this cirrus.

Stomach, destitute of cæca; œsophagus 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; vesiculæ seminales extending into the prosoma. I noticed the ovigerous fræna 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 by the calcareous basis,—by the manner in which the scuta and terga are locked together,—by the two little fans of muscle attached to near the basal points of the terga,—and perhaps by some of the characters of the trophi; nevertheless, this affinity is far from being well-marked, and I think is hardly so plain as in [Pollicipes mitella].