DETERMINATION OF GENERIC TYPES, ETC. 55 



Some authors believe that types should be confined entirely to spe- 

 cies personally examined by the author of the genus, but it will 

 scarcely be possible to carry out this rule. 



C. GENERA FOR WHICH NO TYPE HAS BEEN DEFINITELY 

 SELECTED. 



Unfortunately a very large number of generic names. with which 

 one has to deal at present come under this category. In determining 

 the type we should be governed by certain general principles. It is, 

 however, difficult to lay down any general scheme of precedence in 

 which these principles shall apply, since individual cases may be 

 influenced by considerations of a practical nature. Naturally it would 

 be a desideratum if the subjective element were entirely eliminated in 

 such matters, but it is doubtful whether it is practical to insist upon 

 this point. 



11. COLLECTIVE BIOLOGICAL GROUPS REQUIRING NO TYPE SPECIES. 



RULE. Certain biological groups which have been distinctly proposed as 

 collective groups, but not as systematic units of generic rank, may be treated 

 for convenience as if they were genera, but they require no type species. 



Certain so-called genera have been more or less distinctly proposed 

 as unnatural collective groups in which to place forms which have not 

 3 7 et reached stages in development permitting a definite generic deter- 

 mination. As well-known examples may be mentioned Agamodisto- 

 mum, Amphistomulum, etc. These groups can best be recognized in 

 their original sense, but they should have no type designated for them, 

 and they should not compete with true generic names in connection 

 with the law of priority. 



As examples of this kind cited in the present list may be mentioned 

 the following: 



Agamomermis Stiles, 1903, distinctly proposed as an artificial collective group for 

 immature Mermithidse which can not be definitely determined generically until the 

 adult stage is known. 



Agamonema Diesing, 1851, can be interpreted as a group of the same kind, for 

 immature nematodes, especially of fish. 



Agamonematodum Diesing, 1861, also can be interpreted in the same way. 



Dubium Diesing, 1851, is apparently intended as a group of the same nature. 



Merinthoidea and Merinthoidum Krsemer, 1853, were distinctly proposed as ' ' cache- 

 desordre" for worms resembling Filaria, Gordius, and Mermis. 



Nematoideum Diesing, 1851, is apparently used in the same sense, namely, as a 

 purely collective, indefinite group. 



Collective groups of this kind are of course unnatural, but they are 

 nevertheless convenient, for they enable an international specific 

 nomenclature for certain forms without recourse to classifying worms 

 in an uncertain manner in genera which have a more or less definite 

 status. 



