80 INVERTEBRATE PALAEONTOLOGY 



Cambrian, the less-branched Tetragraptids and Didymo- 

 graptids of the Lower and Middle Ordovician, the 

 Diplograptids of the Middle and Upper Ordovician, and 

 the Rastritids and Monograptids of the Silurian. A 

 mere list of the Graptolite species selected as zonal in- 

 dices is sufficiently formidable ; but when attempts to 

 recognize and distinguish specimens, or even drawings, 

 of the various species are made by students untrained 

 in methods of palaeontological research, the founda- 

 tions of belief are shaken, and all interest founders in a 

 whirl of bewilderment. Agitating problems as to the 

 difference between Omphalotrochus discors and O. rugosus 

 (species beloved of the examiner) have little real value, 

 even if solved, since both forms occur together in the 

 Wenlock Limestone ; while even the family distinction 

 between Actinocrinidae and Amphoracrinidae has no 

 greater stratigraphical significance. Is it not more 

 useful, and less provocative of "cramming," to show 

 that the wrinkled Turbinidae abound in the Silurian, 

 and the smooth Euomphalidae in the Carboniferous ; 

 while the Actinocrinoida are wholly characteristic of 

 the latter period ? The most elementary student ought 

 to know that Ammonites are not found in Cainozoic 

 rocks ; but he ought not to know the difference between 

 Dactylioceras commune and Peronoceras annulatum. If 

 the latter detail has been forced into his unprepared 

 mind, some point of more general application and 

 greater importance must have been omitted or ejected. 



(B) PHYSIOGRAPHICAL 



Indications afforded by fossils as to the conditions pre- 

 valent at the time and place of their life are hardly less 

 important than their chronological evidence. Although 

 it is unsafe to assume that past members of a group of 



