RANGE IN TIME expresses the distribution of a species or group through the fossiliferous beds of the earth's crust.
RETINA .-- The delicate inner coat of the eye, formed by nervous filaments spreading from the optic nerve, and serving for the perception of the impressions produced by light.
RETROGRESSION .-- Backward development.

When an animal, as it approaches maturity, becomes less perfectly organised than might be expected from its early stages and known relationships, it is said to undergo a RETROGRADE DEVELOPMENT or METAMORPHOSIS.
RHIZOPODS .-- A class of lowly organised animals (Protozoa), having a gelatinous body, the surface of which can be protruded in the form of root-like processes or filaments, which serve for locomotion and the prehension of food.

The most important order is that of the Foraminifera.
RODENTS .-- The gnawing Mammalia, such as the rats, rabbits, and squirrels.

They are especially characterised by the possession of a single pair of chisel-like cutting teeth in each jaw, between which and the grinding teeth there is a great gap.
RUBUS .-- The bramble genus.
RUDIMENTARY .-- Very imperfectly developed.
RUMINANTS .-- The group of quadrupeds which ruminate or chew the cud, such as oxen, sheep, and deer.

They have divided hoofs, and are destitute of front teeth in the upper jaw.
SACRAL .-- Belonging to the sacrum, or the bone composed usually of two or more united vertebrae to which the sides of the pelvis in vertebrate animals are attached.
SARCODE .-- The gelatinous material of which the bodies of the lowest animals (Protozoa) are composed.
SCUTELLAE .-- The horny plates with which the feet of birds are generally more or less covered, especially in front.
SEDIMENTARY FORMATIONS .-- Rocks deposited as sediments from water.
SEGMENTS .-- The transverse rings of which the body of an articulate animal or annelid is composed.
SEPALS .-- The leaves or segments of the calyx, or outermost envelope of an ordinary flower.