GLOSSARY. 517 



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 

 filaments spreading from the optic nerve and serving for the 

 perception of the impressions produced by light. 



Retrogression. — Backw^ard 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 retrog^xide deiielopment or metamorpliods. 



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 locomo- 

 tion 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 



generally more or less covered, especially in front. 

 Sedimentary Formations. — Rocks deposited as sediments from 



water. 

 Segments. — The tranverse 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 



belonging to the earlier part of the Palseozoic series. 

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



performance of a particular function. 

 Spinal Chord, — 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 to the various 



organs of the body. 



