AMBULACRA (Lat. ambulacrum, a place for walking).  The perforated spaces or “avenues” through which are protruded the tube-feet, by means of which locomotion is effected in the Echinodermata.

AMMONITIDAE.  A family of Tetrabranchiate Cephalopods, so called from the resemblance of the shell of the type-genus, Ammonites, to the horns of the Egyptian God, Jupiter-Ammon.

AMORPHOZOA (Gr. a, without; morphe, shape; zooen, animal).  A name sometimes used to designate the Sponges.

AMPHIBIA (Gr. amphi, both; bios, life).  The Frogs, Newts, and the like, which have gills when young, but can always breathe air directly when adult.

AMPHICYON (Gr. amphi, both—­implying doubt; kuon, dog).  An extinct genus of Carnivora.

AMPHILESTES (Gr. amphi, both; lestes, a thief).  A genus of Jurassic Mammals.

AMPHISPONGIA (Gr. amphi, both; spoggos, sponge).  A genus of Silurian sponges.

AMPHISTEGINA (Gr. amphi, both; stege, roof).  A genus of Foraminifera.

AMPHITHERIUM (Gr. amphi, both; therion, beast).  A genus of Jurassic Mammals.

AMPHITRAGULUS (Gr. amphi, both; dim. of tragos, goat).  An extinct genus related to the living Musk-deer.

AMPLEXUS (Lat. an Ambrace).  A genus of Rugose Corals.

AMPYX (Gr. ampux, a wreath or wheel).  A genus of Trilobites.

ANARTHROPODA (Gr. a, without; arthros, a joint; pous, foot).  That division of Annulose animals in which there are no articulated appendages.

ANCHITHERIUM (Gr. agchi, near; therion, beast).  An extinct genus of Mammals.

ANCYLOCERAS (Gr. agkulos, crooked; ceras, horn).  A genus of Ammonitidoe.

ANCYLOTHERIUM (Gr. agkulos, crooked; therion, beast).  An extinct genus of Edentate Mammals.

ANDRIAS (Gr. andrias, image of man).  An extinct genus of tailed Amphibians.

ANGIOSPERMS (Gr. angeion, a vessel; sperma, seed).  Plants which have their seeds enclosed in a seed-vessel.

ANNELIDA (a Gallicised form of Annulata).  The Ringed Worms, which form one of the divisions of the Anarthropoda.

ANNULARIA (Lat. annulus, a ring).  A genus of Palaeozoic plants, with leaves in whorls.

ANNULOSA (Lat. annulus).  The sub-kingdom comprising the Anarthropoda and the Arthropoda or Articulata, in all of which the body is more or less evidently composed of a succession of rings.

ANOMODONTIA (Gr. anomos, irregular; odous, tooth).  An extinct order of Reptiles, often called Dicynodontia.

ANOMURA (Gr. anomos, irregular; oura, tail).  A tribe of Decapod Crustacea, of which the Hermit-crab is the type.