484 GLOSSARY. 



Ramus. — One half of the lower jaw in the Mammalia. The portion 

 which rises to articulate with the skull is called the ascending 

 ramus. 



Radicle. — The minute root of an embryo plant. 



Range. — The extent of country over which a plant or animal is 

 naturally spread. 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 fila- 

 ments 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 organized 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 organized 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 characterized 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. 



Scutell^:. — The horny plates with which the feet of birds are gen- 

 erally 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. They are usually green, but sometimes 

 brightly colored. 



Serratures. — Teeth like those of a saw. 



Sessile. — Not supported on a stem or footstalk. 



Silurian System. — A very ancient system of fossiliferous rocks be- 

 longing to the earlier part of the Palaeozoic series. 



Specialization. — The setting apart of a particular organ for the 

 performance of a particular function. 



Spinal Cord. — The central portion of the nervous system in the 

 Vertebrata, which descends from the brain through the arches of 

 the vertebrae, and gives off nearly all the nerves tp the various 

 organs of the body. 



