356 THE CRETACEOUS ROCKS OF BRITAIN. 
following genera, occurring in the Lower Chalk and now existent, 
all range from shallow water to depths of over 1,000 fathoms : 
Aporrhais, Cerithiopsis, Dentalium, Emarginula, Fusus, Natica, 
Seal aria, Solarium, Trochui, and Voluta. Taking those species 
of these genera which are found between 100 and 1,000 fathoms, 
I have calculated the average depth at which they occur on each side 
of the Atlantic on the authority of W. H. Dali* for the American 
side, and of Locardf for the European side. On the American 
side the average depth comes out at about 300 fathoms, and on the 
European side at 263 fathoms. But this result is of little value, 
because there is such great variation in the range of the species 
belonging to these genera, some of them only occurring in small 
depths, and others occurring at or ranging to the greater depths. 
The only way to obtain a more useful result would be to compare 
the Cretaceous and recent forms species by species, to select those 
of the latter which most nearly resemble the former, and then to 
set down the average depth at which these representative species 
are found in modern seas. Such a comparison would require a 
much more special and detailed study of the fossil and recent shells 
than I have time or opportunity for. 
Larnellibranchiata . — The remark just made with regard to the 
Gasteropods holds good also concerning the Lamellibranelis, Most 
of the genera which are found in the Chalk Marl (zone of Ammo¬ 
nites varians ) occur also in the Upper Gault, and even some of the 
species are the same, or represented by slightly different varieties, 
such are Area carinata, Nucula pectinata, Cardita tenuicosta, 
Lima globosa, Pecten orbicularis, Neithea quinquecostata, and Pli- 
eatula in flat a. I think it may be said that there are no species 
in the Chalk Marl which could be regarded as indicating a greater 
depth of water than any in the Upper Gault. 
There are, moreover, some genera which do not at the present 
day extend to any great depth. Thus Ostrea does not now extend 
beyond 300 fathoms, Pinna does not range beyond 200, nor Avicula 
beyond 350, Tellina and Cardita do not range beyond 600, nor 
Spondylus beyond 640. This, however, is negative evidence, and 
there is no reason why Cretaceous species of Ostrea and Pinna 
should not have ranged into deeper water than 300 fathoms, for 
Mr. E. A. Smith cites many recent species which live both in shallow 
and deep water, remarking that, “ so far as our observations have 
reached the shells appear to be very little affected by the difference 
of the depth or the nature of the bottom.” J 
The above remarks refer exclusively to the zone of Am. varians 
in the east and south-central parts of England; the fauna of the 
arenaceous beds which appear to represent the Chalk Marl in Devon¬ 
shire exhibit quite a different facies. These beds contain a number 
* Catalogue of Marine Mollusks of the South-East coast of the United 
States,' 18S9. ; ■ ' 
t Ooquilles Marines au large des cotes de Trance, 1899.- ... . . .. 
X Challenger Reports (Lamellibranchiata) y Xo\. .xiii., p. 5. - , .. 
