HYDROBRANCHIATA. Bl. The first section of the order Gasteropoda, Lam. containing Molluscs which breathe water only; divided into the families Tritoniana, Phyllidiana, Semi-phyllidiana, Calyptracea, Bullæana, and Aplysiana. HYGROMANES. Fer. A sub-division of Helix, containing H. limbata, Auct. &c. Gray's Turton, p. 143. HYRIA. Lam. A genus composed of species of Nayades, distinguished by their alated dorsal margins, and lamellated lateral teeth. South America. HYRIA corrugata, fig. 143, Hyria Syrmatophora, fig. 144. HYRIDELLA. Sw. A genus of "Hyrianæ," Sw. described as differing from HYRIA, Auct. in having a cardinal as well as a lateral tooth in each valve. Sw. p. 380. HISTRIX. Humph. RICINULA, Auct. JANERA. Schum. A genus composed of species of Pecten, Auct. having oblique plicæ or calli on each side of the ligamentary pit. _Ex._ P. plica, fig. 172. Decadopecten, Rüppell. JANTHINA. Auct. (_Janthum_, a violet.) _Fam._ Neritacea, Lam. Oxystomata, Bl.--_Descr._ Sub-globose, thin, fragile; spire short, consisting of few whorls; aperture angulated, at the anterior junction of the inner and outer lips; columella tortuous, contiguous to the axis; outer lip thin, sinuated in the centre.--_Obs._ The shells composing this genus are celebrated for their beautiful purple colour. The animal possesses a small vesicular process, which keeps it floating on the surface of the water; it exudes a purple secretion when irritated. It is occasionally floated on to the shores of most temperate and tropical countries. Fig. 333, J. Fragilis. JATARONUS. Adanson. CHAMA, Auct. IBERUS. Montf. CAROCOLLA, Lam. IBLA. Leach. _Fam._ Pedunculated Cirripedes, Lam.--_Descr._ Four valves, posterior pair elongated, anterior pair short, triangular; pedicle cylindrical, contracted at the base, hairy.--_Obs._ I. Cuveriana (fig. 40) is brought from Kangaroo Island. ICTHYOSARCOLITES. Desmarest. _Fam._ Ammonacea, Lam.--_Descr._ Chambered, slightly arcuate, laterally compressed; septa simple, leaving triangular articulations imbricated like the thick muscles of a fish. JESITES. Montf. A minute fossil resembling GALEOLARIA. ILOTES. Montf. ORBICULINA, Bl. A genus of microscopic Foraminifera. IMBRICARIA. Schum. CONOHELIX, Sw. IMBRICATED. (_Imbrex_, a tile.) A shell is said to be imbricated when the superficial laminæ are arranged over each other in the manner of tiles. IMPERATOR. Montf. A genus composed of species of the genus TROCHUS, Auct. with whorls angulated and stellated, having an umbilicus. _Ex._ T. Imperialis. Some of the shells named Imperator in the British Museum belong to the genus Calcar, Montf. having no umbilicus. IMPRESSION. See MUSCULAR IMPRESSION. INCRASSATED. (_Crassus_, thick.) Thickened, as the hinge of Glycimeris, fig. 67.