THE RULES OF NOMENCLATURE 371 



times separately published, sometimes forming parts of 

 scientific journals, in which only some small groups of 

 fossils, or the fossils from some one locality or district, 

 are described. 



Thirdly, there are various monographs, such as those 

 published by the Palaeontographical Society of London, 

 which aim at a complete description (with revision of all 

 previous publications on the subject), either of some 

 great group as the Brachiopoda, for all geological 

 systems, or for some one system (as the Cretaceous 

 Lamellibranchs or the Carboniferous Trilobites), or, 

 more rarely, the whole fauna of some one system or 

 formation. Such monographs, if properly prepared, are 

 of enormous value to students, as they sum up all that is 

 known up to the date of their publication, and only very 

 special investigators need go behind them to earlier 

 works. These three categories are fairly distinct, but 

 it is not always possible sharply to separate them. 



Fourthly, there are catalogues and indexes which, 

 without themselves describing any new genera or species, 

 enable students to find with the least labour those works 

 where they are described. 



Some of these monographs, etc., have already been 

 noted in the short bibliographies appended to each 

 chapter. It remains to list a few of the works of more 

 general kind. Fuller bibliographies of particular groups 

 of fossils will be found in some of these. 



General Bibliography. 



I. INDEXES. 



BRONN, H. G. Index Palaeontologicus (1848-49). 

 Records every species of fossil plant and animal named 

 up to date of publication, with references to author, 

 synonymy, geological age,^etc. 



