HYDROIDA (Gr. hudra; and eidos, form).  The sub-class of the Hydrozoa, which comprises the animals most nearly allied to the Hydra.

HYDROZOA (Gr. hudra; and zooen, animal).  The class of the Coelenterata which comprises animals constructed after the type of the Hydra.

HYMENOPTERA (Gr. humen, a membrane; pteron, a wing).  An order of Insects (comprising Bees, Ants, &c.) characterised by the possession of four membranous wings.

ICHTHYODORULITE (Gr. ichthus, fish; dorus, spear; lithos, stone).  The fossil fin-spine of Fishes.

ICHTHYOPTERYGIA (Gr. ichthus; pterux, wing).  An extinct order of Reptiles.

ICHTHYORNIS (Gr. ichthus, fish; ornis, bird).  An extinct genus of Birds.

ICHTHYOSAURIA (Gr. ichthus; saura, lizard).  Synonymous with Ichthyopterygia.

IGUANODON (Iguana, a living lizard; Gr. odous, tooth).  A genus of Deinosaurian Reptiles.

INCISOR (Lat. incido, I cut).  The cutting teeth fixed in the intermaxillary bones of the Mammalia, and the corresponding teeth in the lower jaw.

INEQUILATERAL.  Having the two sides unequal, as in the case of the shells of the ordinary bivalves (Lamellibranchiata).  When applied to the shells of the Foraminifera, it implies that the convolutions of the shell do not lie in the same plane, but are obliquely wound round an axis.

INEQUIVALVE.  Composed of two unequal pieces or valves.

INOCERAMUS (Gr. is, a fibre; keramos, an earthen vessel). 
An extinct genus of Bivalve Molluscs.

INSECTA (Lat. inseco, I cut into).  The class of articulate animals commonly known as Insects.

INSECTIVORA (Lat. insectum, an insect; voro, I devour).  An order of Mammals.

INSECTIVOROUS.  Living upon Insects.

INSESSORES (Lat. insedeo, I sit upon).  The order of the Perching
Birds, often called Passeres.

INTERAMBULACRA.  The rows of plates in an Echinoid which are not perforated for the emission of the “tube-feet.”

INTERMMAXILLAE or PRAEMAXILLAE.  The two bones which are situated between the two superior maxillae in Vertebrata.  In man, and some monkeys, the praemaxillae anchylose with the maxillae, so as to be irrecognisable in the adult.

INVERTEBRATA (Lat. in, without; vertebra, a bone of the back).  Animals without a spinal column or backbone.

ISOPODA. (Gr. isos, equal; podes, feet).  An order of Crustacea in which the feet are like one another and equal.

KAINOZOIC (Gr. kainos, recent; zoe, life).  The Tertiary period in Geology comprising those formations in which the organic remains approximate more or less closely to the existing fauna and flora.