CRETACEOUS PERIOD. 477 



showing their relations to species of the corresponding divisions in Europe, is from a 

 paper by Meek & Hayden : — 



Earlier Cretaceous W. of Miss. R. Lower or Gray Chalk in Europe. 



Ammonites vespertinus Mort. occurs in Austria. 



A. percarinatus H. & M. probably identical with A. Woolgari Mantell. 



Scaphites Warreni M. & H. scarcely distinct from *S. cequalis, Sowerby. 



S. larvceformis M. & H. same type as S. oequalis. 



Nautilus eleyans, var. scarcely distinct from N. elegans Sowerby. 



Inoceramus latus (?) appears to be the same as /. latus Mantell. 



I. problematicus cannot be distinguished from /. problematicut 



Schlot.; reported also from the Upper Green- 

 sand of Europe. 



Species common to the Later Cretaceous of America and the Upper or White Chalk 

 of Europe: Saurocephalus lanciformis Harlan, Lamna acuminata Ag., Belemnitella 

 mucronata, Neithea Mortoni, Ostrea larva, Gryphoza lateralis, Gryphosa vesicularis, 

 Nucleolites crucifer Mort. The Gryphaza vesicularis is supposed by some to occur also 

 in the Upper Green sand and the LoAver or Gray Chalk; but the form found in these 

 lower portions is regarded by other authorities as a distinct species. 



Genera of the Later Cretaceous of America not yet found below the White Chalk 

 of Europe : Mosasaurus, Saurocephalus, Callianassa, Pleurotoma, Fasciolaria, Cyproza 

 Pulvinites, Cassidulus. There are also in the American Later Cretaceous the two 

 genera Pseudobuccinum and Xylophaga (?), which have not yet been found as low as 

 the Cretaceous in Europe. 



3. General Observations. 



1. Origin of the Chalk and Flint. — From the absence of vegetable 

 remains and earthy ingredients, the abundance of sponges, and the 

 relations of the fossils to species now found in the deep Atlantic, it is 

 supposed that the Chalk was formed at a distance of some miles from 

 shore, where the water was at least several hundred fathoms deep. 

 The abundance of Rhizopod shells, as already stated, suggests that 

 these were the main material ; and the recent observation that the 

 lead in deep-sea soundings over the north Atlantic has often brought 

 up sand composed almost wholly of minute Rhizopods, as first an- 

 nounced by Bailey, sustains the conclusion. These shells are like 

 grains of sand in size, and are, therefore, ready for consolidation into a 

 compact rock, needing no previous trituration by way of preparation ; 

 and thus they are especially fitted for making deep-water limestones. 

 In the Atlantic, the mud of the bottom, where not over 2,500 fathoms 

 in depth, is often eighty-five per cent, the shells of Globigerina, the 

 kind of Rhizopod represented in Fig. 171 ; the most common species 

 is G. bulloides. The softness or imperfect aggregation of Chalk is 

 probably due to this origin, and particularly to the fact that each grain 

 is a cellular shell, or collection of air-cells, instead of solid. The coral 

 reefs of the Pacific do not under ordinary circumstances give rise to 

 chalk. The only chalk known in coral regions is on Oahu, at the foot 

 of an extinct volcanic cone ; and there it is probable that warm waters 



