| A | B |
|---|
| atom | The smallest particle in an element. |
| geological time scale | A record of the geological events and life forms in Earth's history. |
| era | One of the three long units of geological time between the Precambrian and the present. |
| period | One of the units of geological time into which geologists divide eras. |
| comet | A loose collection of ice, dust and samll rocky paticles, typically with a long, narrow orbit of the sun. |
| continental drift | The hypothesis that the continents, with cold winters and warm or hot summers. |
| invertebrate | An animal without a backbone. |
| vertebrate | An animal with a backbone. |
| reptile | A vertebrate with scaly skin that lays eggs with tough, leathery shells. |
| amphibian | A vertebrate that lives part of its life on land and part in water. |
| mass extinction | When many types of living things become extinct at the same time. |
| mammal | A warm-blooded vertebrate that feeds its young milk. |
