78 INVERTEBRATE PALAEONTOLOGY 



marked by the presence of Graptolites, Trilobites, and 

 certain orders and suborders of Brachiopods that are 

 either restricted within, or rare beyond, its limits. 

 Similarly the Upper Palaeozoic (Deuterozoic) contains 

 remains of plants, Fishes, Molluscs and groups of 

 Brachiopods that are ordinally distinct from those 

 common in the preceding or succeeding eras. The 

 Mesozoic is often called the " age of Reptiles," but some 

 orders of Echinoids, Brachiopods and Molluscs (especially 

 Cephalopods) are equally characteristic. The Cainozoic 

 is the " age of Mammals." However, neither these 

 great divisions, nor minor ones, can be defined with 

 accuracy until Palaeontology has indicated the limits 

 of hemerae, the units of geological time. 



(II) CHARACTERISTIC FOSSILS 



Acquaintance with fossils characteristic of the various 

 periods is usually required of students of Geology, and 

 is essential for those proposing to investigate stratigraphy 

 in the field. The former sufferers deserve considerable 

 sympathy, since the exigencies of examination courses 

 rarely leave time for adequate inquiry into the biological 

 characters of the fossils whose appearance, names and 

 range they are expected to learn ; while both classes of 

 workers are often troubled by the varied opinions of 

 authorities as to the particular species that are 

 " characteristic." Moreover, Nature often adds to the 

 confusion by local or sporadic distribution of common 

 forms. It is usual, and correct, to describe Hemicidaris 

 intermedia as a characteristic fossil of the Corallian ; but 

 while a worker in Wiltshire might perhaps collect 

 specimens of that species by the bag-full, another in 

 Berkshire, searching at the same horizon and in litho- 

 logically similar beds, would never find a trace. 



