182 PALEONTOLOGY 



1. Incomplete exploration of large areas of the earth's 

 surface of which indeed three-quarters is practically 

 preserved from exploration by a covering of deep sea. 



Within the last twenty years, exploration of new areas 

 has actually revealed the ancestors of what had been 

 cryptogenetic types (not indeed of ammonites, but of 

 trilobites and mammals), and justified Neumayr's migra- 

 tion-theory. 



2. Destruction of fossiliferous strata by denudation. 

 Not only have large parts of some continents been swept 

 bare of masses of strata that once covered them (as 

 proved by isolated patches preserved by some accident), 

 but it is now realized that among shallow-water deposits 

 there has been much " pene-contemporaneous " erosion, 

 and that in what looks like a thick conformable series of 

 sediments there may be many gaps due to this cause. 



3. New stocks begin with small members which are 

 liable to be overlooked in collecting or in subsequent 

 investigation ; and even larger forms, if rare (and they 

 may be rare if the stock has not yet attained dominance), 

 are liable to be neglected. 



DIBRANCHIATA. 



The most familiar fossils of this order are those known 

 as Belemnites, the most typical of which are found in the 

 Jurassic and Lower Cretaceous systems, though forms 

 but little different also occur in the Upper Triassic and 

 Upper Cretaceous. A typical belemnite shell consists of 

 (i) a phragmocone (chambered shell) which is an orthocone 

 or cyrtocone with a calcareous globular protoconch, 



