o54 the cretaceoHs rocks oE Britain. 
a few Cephalopoda. Of Gasteropoda only two or three genera have 
been found, and of these only Solariella and Pleurotomaria are 
widely distributed. 
The rarity of Gasteropoda might seem to indicate that the water 
was fairly deep, for at the present day they are most abundant 
in shallow water, and in depths of less than 500 fathoms ; still 
there are many genera, such as Buccinum, Fusus, Trophon, 
Scalaria, Chemnitzia, Cerithiopsis, Natica, Solarium, Turbo, 
Trochus,said Emarginula, which range below 1,000 fathoms, and 
a few even below 2,000 fathoms. Most of these certainly existed in 
the Cretaceous period, since they occur in the Gault, and we 
cannot account for their absence in Middle Chalk by supposing 
that they have been destroyed since its upheaval, for in that case 
their casts would remain as thev do in the Lower Chalk and in 
the Chalk Rock. 
The absence of the phytophagous genera could hardly be due 
to the want of vegetable food on which to support existence, 
because modern deep sea species are supposed to feed on the 
pelagic algay which fall from the surface waters, and on the plant 
remains which are floated out from the land by currents. There 
is no reason to suppose that either of these sources of food was 
wanting in the waters of the Middle Chalk. 
The few casts that do occur show that Gasteropods did exist, 
and we can only suppose that the shells were easily destroyed by 
solvents, and that most of such as existed were disintegrated 
during the formation of the chalk, just as Pteropod and Hetero¬ 
pod shells are disintegrated now at depths of more than 1,000 
fathoms. 
Of Lamellibranchs, the commonest belong to the genera Ostrea, 
Neithea, Spondylus, and Inoceramus. The last is extinct, and 
in modern seas the two first do not range beyond 300 fathoms, 
but Spondylus goes to 640. There is again a remarkable absence 
of those genera which are now characteristic of greater depths, 
but it is possible that such shells were actually present on the 
sea-floor while the material of the Middle Chalk was in process 
of formation, and that they were so thin and perishable that they 
soon fell to pieces and went to increase the bulk of the chalky 
mud itself. 
Brachiopoda are as a rule the commonest fossils in the 
Middle Chalk, Rhynchonella Cuvieri and Terebratulina lata being 
often present in large numbers, and as they belong to types which 
have not undergone much change, their value for comparison with 
modern species is greater. The principal genera are Rhynchonella, 
