356 PALAEONTOLOGY 



inevitably differ: as such differences constantly occur, 

 this definition recognizes the facts of the case. 



A genus may be defined either as a collection of species 

 having certain features in common (the morphological defini- 

 tion) or as a collection of species believed to be derived from 

 the same immediate ancestral stock (the genetic definition). 

 The difference between these definitions has been 

 illustrated under the Graptolites. 



As a rule, each of the categories named above consists 

 of several of the next category below, but it may consist 

 of only one e.g., a genus may include only one species, 

 a family may include only one genus. 



If the categories named are not enough for satisfactory 

 classification in any particular case, intermediate grades 

 may be intercalated, denoted by the prefix sub- ; thus an 

 order may be divided into several sub-orders, each con- 

 taining several families. Still further grades may some- 

 times be necessary : they are denoted by the prefix super- : 

 thus a sub-order may be divided into super-families, and 

 these into families. The term variety is generally used 

 in place of sub-species. 



The system of nomenclature for species now universally 

 adopted is that of the Swedish naturalist Linne (latinized 

 as Linnaeus) and consists in denoting every species by a 

 double name : hence it is often called the binominal 

 system. Thus " Dalmanites caudatus " is the specific 

 name of a certain species of Trilobite, and is made up 

 of the generic name " Dalmanites " (the name of the 

 genus to which this species belongs), and the trivial 

 name " caudatus " which distinguishes this species from 

 others of the same genus. 



A generic name is always a single word, either a Latin 

 noun or a word treated as such. In the early days of the 

 Linnaean nomenclature Latin or latinized names of 

 animals were plentifully at hand, but with the increase 



