GEOLOGICAL PALAEONTOLOGY 79 



Similarly Conchidium knighti is not merely "charac- 

 teristic " of the Ludlow Limestone, but often constitutes 

 much of its bulk ; at Weo Edge, in Shropshire, single 

 specimens can hardly be collected owing to the con- 

 gestion of shells, but at West Knowle, about a mile 

 away, the same bed of Limestone has not yielded a 

 single example. 



Such familiar species as those mentioned above have 

 the merit of abundance in some localities (as well as 

 that of restricted vertical range), but the same cannot 

 be said of many types whose names figure in some lists 

 of characteristic fossils. It is a fairly safe assumption 

 that, granted sufficient knowledge and minute study, 

 any fossil can supply precise stratigraphical evidence, 

 whether it is common or scarce. To a casual in- 

 vestigator, the three abundant Echinoids of the Upper 

 Chalk, Micraster, Echinocorys and Conulus, seem to per- 

 sist with very little change throughout that stage. But 

 to one well-acquainted with Cretaceous stratigraphy or 

 Echinoid morphogeny, differences appear in specimens 

 collected from horizons only a few feet apart. It is 

 unlikely that the zonal progress recognized, and used, 

 in these forms is peculiar to any special group of 

 organisms. Nevertheless, appreciation of morphological 

 refinements is impracticable for average geological 

 students, so that discrimination of well-marked species 

 and genera becomes necessary. 



The author is strongly of the opinion that recognition 

 of species should be left for students of Palaeontology, 

 or those who carry stratigraphical studies beyond a 

 preliminary stage. The chronological distribution of 

 families and genera will give ample precision for ele- 

 mentary needs. Unless detailed work is in prospect, it is 

 sufficient to know that Graptolites are characteristic of 

 the Lower Palaeozoic the dendroid types of the Upper 



