122 Belemnitella Americana and Mucronata [320 



known fauna, from a great thickness of Senonian, in which all 

 stages have been differentiated. In Morocco there is less 

 known about the Senonian, but in all of these occurrences 

 B. mucronata is noticeable only by its absence. In Egypt the 

 Campanian and Maestrichtian are very well developed, the 

 latter becoming thicker as it passes southward. In the north, 

 it contains oysters and Exogyra overwegi; to the south, where 

 the facies is one of ferruginous sands, and gypseous and salt- 

 bearing clays, attaining a thickness of 150 meters, Zittel has 

 recorded a very large fauna, with no B. mucronata. 



Such negative evidence could be continued indefinitely, 

 always affording striking corroboration of the evidence af- 

 forded by its occurrence. I do not see how it is possible to 

 escape the conclusion that there is undoubtedly some connec- 

 tion between the conditions under which glauconite and 

 chalk were deposited, and endurable life conditions for B. 

 mucronata and B. americana. Also, the abruptness with 

 which they disappear when any other facies occurs seems 

 to indicate a very restricted power of adaptation. Does their 

 occurrence in glauconitic sands and chalk indicate the maxi- 

 mum effort of this limited power of adaptation under two dif 

 ferent conditions of life, or do the glauconite and chalk mean 

 practically identical conditions of deposition? The latter, in 

 the light of the most recent interpretation of these facies, is 

 to be regarded as the correct conclusion. 



The bathymetric conditions under which chalk was laid 

 down have been the source of much speculation, which has 

 gradually undergone a most marked change. The venerable 

 and orthodox idea is that the chalk is a deposit formed in a 

 large ocean at great depths, that it is an abyssal deposit com- 

 parable to the Globigerina-ooze of recent seas. The large pro- 

 portion of Foraminifera was regarded as proof of this. Cayeux 

 (14, p. 523-4) give a resume of the various expressions of 

 opinion on the origin of chalk, dating from 1833 to the end of 

 the nineteenth century. Mantell, in 1833, and E. A. C. 

 Austin in 1843, both postulate abyssal conditions. Good- 

 win-Austin in 1858 writes that A. D'Orbigny and E. Forbes 



