Axis

(Gr. axon, a pivot). The second vertebra of the neck, upon which the skull and atlas usually rotate.

Azygous

(Gr. a, without; zugon, yoke). Single, without a fellow.

Bacterium

(Gr. bakterion, a staff). A microscopic organism occurring in fluids containing organic matter, and having a staff-shaped form.

Balanidae

(Gr. balanos, an acorn). A family of sessile Cirripedes, commonly called "Acorn shells."

Baleen

(Lat. baloena, a whale). The horny plates which occupy the palate of the "whalebone" Whales.

Batides

(Gr. batos, a bramble). The family of the Elasmobranchii comprising the Rays.

Batrachia

(Gr. batrachos, a frog). Often loosely applied to any of the Amphibia, but sometimes restricted to the Amphibians as a class, or to the single order of the Anoura.

Belemnitidae

(Gr. belemnon, a dart). An extinct group of Dibranchiate Cepha-lopods, comprising the Belemnites and their allies.

Bicavitary

(Lat. bis, twice; cavus, hollow). Consisting of or possessing two cavities.

Bifid

Cleft into two parts ; forked.

Bilateral

Having two symmetrical sides.

Bimana

(Lat. bis, twice ; manus, a hand). The order of Mammalia comprising Man alone.

Bipedal

(Lat. bis, twice ; pes, foot). Walking upon two legs.

Biramous

(Lat, bis, twice; ramus, a branch). Applied to a limb which is divided into two branches (e.g., the limbs of Cirripedes).

Bivalve

(Lat. bis, twice; valval, folding-doors). Composed of two plates or valves ; applied to the shell of the Lamellibranchiata and Brachiopoda, and to the carapace of certain Crustacea.

Blastoidea

(Gr. blastos, a bud ; and eidos, form). An extinct order of Echinodermata, often called Pentremites.

Bla.stostyle (Gr. blastos, a bud; and stulos, a column). Applied by Prof. Allman to certain columniform zooids in the Hydrozoa which are destined to bear generative buds.

Brachiopoda

(Gr. brachion, an arm; pous, the foot). A class of the Mol-luscoida, often called " Lamp-shells," characterised by possessing two fleshy arms continued from the sides of the mouth.

Brachium

(Gr. brachion, arm). Applied to the upper arm of Vertebrates.

Brachyura

(Gr. brachus, short; oura, tail). A tribe of the Decapod Crustaceans with short tails (i.e., the Crabs).

Bracts

(See Hydrophyllia).

Bradypodidae

(Gr. bradus, slow ; podes, feet). The family of Edentata, comprising the Sloths.

Branchia

(Gr. bragchia, the gill of a fish). A respiratory organ adapted to breathe air dissolved in water.

Branchiate

Possessing gills or branchiae.

Branchifera

(Gr. bragchia, gill; and phero, I carry}. A division of Gasteropodous Molluscs, in which the respiration is aquatic, and the respiratory organs are mostly in the form of distinct gills.