88 INVERTEBRATE PALAEONTOLOGY 



conditions. It is usually necessary to determine the 

 bathymetric range of the fauna of each period by various 

 means (largely inductive), and to apply that knowledge 

 to the fossils of the period concerned. In this way 

 interesting conclusions can be reached as to variations 

 in the depth of the Chalk sea. Ammonoids and 

 Gastropods are fairly abundant in the Lower Chalk, 

 become rare in the Middle Chalk, reappear in great 

 numbers in the Chalk-Rock, and practically disappear 

 from the Upper Chalk. Their presence corresponds 

 with the prevalence of relatively shallow water ; and, for 

 the Cretaceous period at least, they may be used as 

 evidence for such conditions. Similarly, Discoidea y an 

 Echinoid ancestral to modern " Cake-Urchins " and 

 "Sand Dollars" (whose distribution is mainly littoral), 

 occurs in the Lower Chalk, and is represented by 

 minute species in the Middle Chalk ; but it is entirely 

 absent from higher zones in this country. In other 

 districts, however, such as Southern France and Northern 

 Africa, the genus and its allies are abundant throughout 

 the Senonian, in deposits of less pelagic characters than 

 the British Upper Chalk. 



(IV) FOSSILS AS EVIDENCE OF CLIMATE 



Uncertainty as to the habits of extinct types of 

 Invertebrates, and virtual restriction of their remains to 

 marine types, make attempts at climatic "reconstruc- 

 tion " of doubtful value when based on such evidence. 

 Although reef-building Corals flourish in warm seas at 

 present, there is no more reason to suppose that their 

 past distribution was similarly determined than to 

 regard Proboscidea as exclusively tropical forms. The 

 Mammoth endures as a lasting warning against assump- 

 tions of this kind. In the case of relatively modern 



