920 ■ PRINCIPLES OF STRATIGRAPHY 



Generic Types. Species upon which genera are based are 

 genotypes* Three kinds of genotypes may be recognized : 



Genoholotype — the original species on which the genus is 

 founded, or the species selected by the autnor from those originally 

 described as the type of the genus. 



Genosyntype — one of a series of species upon which a genus is 

 founded when there is no genoholotype. 



Genolectotypc — a species subsequently selected from the geno- 

 syntypes to represent the genoholotype. 



Selection of the Genotype or Type Species of a Genus. 



Many genera are monotypic, i. e., had only one species when 

 founded, though others may subsequently have been referred to 

 them. The original species upon which the genus is founded in 

 such a case is the true genotype or genoholotype. When a genus 

 is founded on a group of species (heteiotypic) the originator of 

 the genus should select one species as the genoholotype. This has 

 not always been done, especially in the case of the older genera, 

 the genus being founded on a group of species or genosyntypes de- 

 scribed at the same time. It then becomes the duty of the first re- 

 viser of the genus to select the type species (genolectotype) from 

 the original species (or genosyntypes). Two principal methods 

 are used by naturalists in such cases — the first species tnethod and 

 the elimination method. The first of these methods appears to be 

 the simplest one, since the species first described by the author of 

 the genus is taken as the type. It sometimes happens, however, 

 that the first species is not the most typical of the genus as defined 

 by the author, or it may have been subsequently separated from the 

 other species and perhaps placed in a genus by itself, the diagnosis 

 of which dififers from the original one, or is more circumscribed 

 than it. In such a case it is the practice to choose the genolectotype 

 from the remaining species of the original group. Often several 

 sections have been separated from the original group and placed in 

 distinct genera. By this process of elimination the genotype thus 

 becomes restricted to the remaining species (genosyntypes), one of 

 which must be selected. This selection is to be done by the first 

 reviser of the old genus and his designation of the genolectotype 

 will stand. Occasionally it may happen that all the original species 

 have been removed to new genera, in which case the last one so 



* This name has recently been employed by zoologists and botanists in a very 

 different sense (see Osborn-19). 



