494 THE DEPTHS OF THE SEA. [chap. x. 



horny substance enclosing the greater part of the 

 curved spiral shell. Now if the recent Spirilla had 

 been weighted with such a rostrum it would probably 

 have remained up to the present time utterly unknown 

 to ns. It is unwise to prophesy, but I certainly 

 look upon some form allied to Splruliroslris as one 

 of the most likely spoils of the deep sea, From the 

 Tertiaries we pass to the Cretaceous forms, and find 

 in Belemnitella the chambered shell straightened and 

 reduced, and the ' guard ' greatly increased in size. 

 If Belemnites were deep-sea animals, as seems very 

 probable, and if any of them still exist, — from the 

 form and weight of their shells it is scarcely possible 

 that they should ever be thrown up on the shore, 

 and without deep-sea dredging they might remain 

 for ever unknown. I merely mention this to show 

 that it is by no means safe to base even what little 

 argument might rest upon it, upon the absence 

 at the present time of all representatives of the 

 cretaceous cephalopodous fauna. 



The gasteropods, with comparatively few excep- 

 tions, range from the shore to a depth of 100 to 

 200 fathoms, and lamellibranchs become scarcer at 

 a slightly greater depth ; while some orders of bra- 

 chiopods, Crustacea, echinoderms, sponges, and fora- 

 minifera, descend in scarcely diminished numbers to 

 a depth of 10,000 feet. In fact, the bathy metrical 

 range of the various groups in modern seas corre- 

 sponds remarkably with their vertical range in 

 ancient strata. 



A change in the distribution of sea and land in- 

 volving a mere change in the course of an ocean- 

 current miffht modify the conditions of an area for 



