252 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



led to the association of groups of similar species into 

 genera, has given rise to the combination of allied genera 

 into higher groups, which are termed Families. It is 

 obvious that the definition of a family, as a statement of 

 the characters in which a certain number of genera agree, 

 is another morphological abstraction, which stands in the 

 same relation to generic, as generic do to specific abstrac- 

 tions. Moreover, the definition of the family is a statement 

 of the plan of all the genera comprised in that family. 



The family of the Northern crayfishes is termed 

 Potamobiidce ; that of the Southern crayfishes, Par- 

 astaciilce. But these two families have in common all. 

 those structural characters which are special to neither; 

 and, carrying out the metaphorical nomenclature of the 

 zoologist a stage further, we may say that the two form 

 a, Tribe — the defm^ion_ojLY^ich^esciibes i the plan which 

 is common to both families. 



It may conduce to intelligibility if these results are put 

 into a graphic form. In fig. 66, A. is a diagram represent- 

 ing the plan of an animal in which all the externally 

 visible parts which are found, more or less modified, in 

 the natural objects which we call individual crayfishes 

 are roughly sketched. It represents the plan of the 

 tribe. B. is a diagram exhibiting such a modification 

 of A. as converts it into the plan common to the whole 

 family of the Parastacidce. C. stands in the same re- 

 lation to the Potamobiidce. If the scheme were thoroughly 

 worked out, diagrams representing the peculiarities of 



