72 INVERTEBRATE PALAEONTOLOGY 



and many families. Similarly the name Rhynchonella 

 is restricted to a few peculiar species ; the original 

 " genus " included almost all the members of the family 

 Rhynchonellidae of modern classification. The struc- 

 tural characters whose discovery has led to dismember- 

 ment of such comprehensive " genera " are often of 

 real importance, but no less frequently difficult to ap- 

 preciate without detailed study. A student of Palaeon- 

 tology for its own sake can, and should, recognize 

 recent generic divisions, and try to check their validity ; 

 but a student of Geology, whose purpose in dealing 

 with fossils is to discover indices of stratigraphical 

 horizons, cannot hope to prove for himself that any 

 generic differences separate such well-known forms as 

 Terebratula sphaeroidalis and T. simplex. It is, of 

 course, possible for him to remember, parrot-wise, that 

 the one is a Sphaeroidothyris while the other is a 

 Pseudoglossothyris, but information gained at second- 

 hand is not knowledge, and, in such a case, is of very 

 doubtful value. For all but specialists, most of the 

 Terebratulidae must be classed under the name Tere- 

 bratula (sens, lat^) ; all Ammonites must be Ammonites 

 still, though no single species can rightly claim such 

 a generic prefix. 



