.1895. OCEAN DEPOSITS, ANCIENT AND MODERN. 393 



Pleuvotomavia alone persisting and presenting a few isolated examples . 

 sparsely scattered throughout the Chalk strata. 



The questions as to the distribution of these larger molluscs, and 

 as to the bathymetric range of littoral or shallow-water animals, 

 especially in their relations to oceanic currents, are of great interest ; 

 but having confined my remarks in this paper to a comparative 

 analysis of two oceanic periods separated by a wide interval of time, 

 I would now merely summarise the general deductions. 



1 . In Upper Cretaceous times the principal gastropod genera, which 

 subsequently became so prominent in the early Tertiaries, were fully 

 established on the shores of the South German Continent, and 

 attained gigantic proportions yet further to the south at Gosau in 

 the Tyrol, and in the Hippuritic Limestones, Outside European 

 areas, gigantic Helcions have been found by Schmidt in Saghalien, 

 and a rich gastropod fauna occurs in similar deposits in India and 

 Natal, including such types as Fasciolavia and Chemnitzia. The rarity 

 of Gastropoda and Dimyaria in the Chalk strata of Britain is due to 

 the physical conditions under which they were deposited. The pre- 

 sence in Ireland of a more abundant molluscan fauna is evidenced by 

 some thirty gastropod specimens from Lisburn and elsewhere, now in 

 the Jermyn Street Museum. The principal genera are Trochus, 

 Turntella, Rostellaria, Turbo, Pleurotoinaria and Scalar ia, which are a 

 further proof that shallower-water conditions prevailed in those 

 localities during Upper Chalk times. 



2. The " Challenger " Expedition has revealed to us the general 

 characters of the bathymetric distribution of the MoUusca, and has 

 shown that, instead of these being divided into several classes differing 

 from each other according to the depth, a few genera only, having 

 many shore-line representatives, are capable of extending their range 

 to the greater depths. The reasons for such extension are at present 

 uncertain, but the fact is clear that the trochoid Gastropoda [Trochus 

 and Bacilissa), Dentalium, Pleurotoma, and the thin-shelled Pelecypoda, 

 Necera, CaUocardia, with Area, have special abilities for overcoming 

 these unfavourable conditions, while Monomyaria are most indepen- 

 dent of external circumstances. 



3. The molluscan fauna from the Upper Cretaceous of Northern 

 Europe has a general distribution markedly similar to that existing in 

 the present oceans. In the Cretaceous strata adjacent to the con- 

 tinental shores, the Gastropoda and Dimyaria have a preponder- 

 ance over the Monomyaria, similar to that observed at the present 

 day, while in the deeper-water strata these latter assume the lead in a 

 most striking manner. Further, many of the genera extending their 

 range under present conditions, more especially Dentalium and Area, 

 are identical in character and habits with those of past ages. The 

 evidence, therefore, tends to establish my thesis, that the distribution 

 of the Mollusca has presented in the main the same features ever 

 since Upper Mesozoic times. 



