1895- OCEAN DEPOSITS, ANCIENT AND MODERN. 391 



our attention. In the upper portion of the Belemnitella mucronata zone 

 at Norwich the following forms have been met with : Emarginulina, 

 Coybida, Chaiiia, Nucula, and Dentalimn. The appearance of these 

 seems to have been the first sign of that change which was about to 

 close the Cretaceous period, and the first indication of that reelevation 

 which was to alter the whole character of palaeontological history. 

 This single gastropod, the scaphopod, and these few dimyarian 

 bivalves seem already to suggest the great alteration in physical 

 conditions which was about to supervene. To these must also be 

 added the large Tevebratida obesa, which occurs both in the Warminster 

 Upper Greensand and the Upper Chalk of Norwich. 



A comparison of the general relations of the molluscan sequence 

 in the past with that existing at the present day seems to establish a 

 certain resemblance and parallelism which should receive the fullest 

 attention from the student of Cretaceous history, and the following 

 facts seem worthy of consideration. The character of the genera from 

 the South German strata, as also of those from the Blackdown Hills, 

 is that of a fauna now living at a depth of less than 30 fathoms. The 

 depression producing the Upper Greensand formation caused the dis- 

 appearance of many gastropod and dimyarian genera, the survivors 

 being similar to those now existing beyond the 30-fathom line. The 

 predominance in the Lower Chalk of such Monomyaria as Pecten ami 

 Lima is evidence of a depth exceeding 300 fathoms ; and this is con- 

 firmed by other evidence (see Pi'oc. Geol. Assoc, May, 1894). From 

 the Middle Chalk period onward are to be found only these 

 ubiquitous types, and those of extinct monomyarian genera ; but the 

 presence of Trochiis, Tuvbo, and Solarium in the Chalk Rock is 

 paralleled at the present day by the same association found at 

 Culebra Island at a depth of 390 fathoms. Referring to the fact, 

 already noted, that the Mollusca of the Upper Chalk are of the same 

 types as those which at the present day extend their range to the 

 greater oceanic depths, and that the faunal characters are so closely 

 paralleled by those of the oceans of the present day, I would submit 

 that the Chalk Sea was a true oceanic depression, and would use this 

 as an argument against the permanence of oceanic basins (see 

 my " Genesis of the Chalk," p. 227). 



Although after careful study it seems evident that those genera 

 which are present in both the Chalk and the now existing oceans 

 have the same character of distribution, it must be admitted that 

 many types of the present day having a wide bathymetrical range had 

 no representatives in Cretaceous times. Pleurotoma is a form of the 

 widest distribution in depths exceeding i ,000 fathoms, yet no traces of it 

 are to be found in the chalks and marls of the Upper Cretaceous period 

 Among the deep-sea Pelecypoda, Neara, Callocardia, and Cryptodon are 

 most abundant, yet neither in this nor in any other land has the Chalk 

 formation yielded a single specimen. Their absence may be due to 

 either of two reasons : one, that the shell-structure is of so delicate a 



