404 GLOSSARY. 



HOLOTHUROIDEA (Gr. holothourion, a kind of zoophyte ; and eidos, 



form). An order of Echinodermata comprising the Trepangs. 

 HOMOCERCAL (Gr. homos, same; kerkos, tail). Applied to the 



tail of Fishes when it is symmetrical, or composed of two 



equal lobes. 

 HYBODONTS (Gr. hubos, curved; odous, tooth). A group of 



Fishes of which Hybodus is the type-genus. 

 HYDROIDA (Gr. hudra, water-serpent; and eidos, form). The 



sub-class of the Hydrozoa, which comprises the animals most 



nearly allied to the Hydra. 

 HYDROZOA (Gr. hudra; and zoVn, animal). The class of the 



Ccclenterata which cqmprises animals constructed after the 



type of the Hydra. 

 HYMENOPTERA (Gr. humen, a membrane; pteron, a wing). An 



order of Insects (comprising Bees, Ants, etc.) characterized 



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 inter- 

 maxillary 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 Perch- 

 ing Birds, often called Passeres. 



INTERAMBULACRA. The rows of plates in an Echinoid which are 

 not perforated for the emission of the " tube-feet. " 



INTERMAXILL^E or PR^:MAXILL/E. 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 irrecognizable in the adult. 



