352 
ELEMENTS OE GEOLOGY. 
Fig. 360. 
the proportion of species common to the different members 
of the Oolite. Between the Lower Oolite and the Lias there 
is a somewhat greater break, for out of 256 mollusca of the 
Upper Lias, thirty-seven species only pass up into the In¬ 
ferior Oolite. 
In illustration of shells having a great vertical range, it 
may be stated that in England some few species pass up 
from the Lower to the Upper Oolite, as, 
for example, Khynchonella obsoleta^ Li- 
thodomus inclusus^ Pholadomya avails^ 
and Trigonia costata. 
Of all the Jurassic Ammonites of Great 
Britain, A, macrocephalus (Fig. 360), 
which is common to the Great Oolite 
and Oxford Clay, has the widest range. 
We have every reason to conclude 
Schipth. One-thMiiatu- that the gaps which occur, both be- 
tween the larger and smaller sections 
of the English Oolites, imply intervals 
of time, elsewhere represented by fossiliferous strata, al¬ 
though no deposit may have taken place in the British area. 
This conclusion is warranted by the partial extent of many 
of the minor and some of the larger divisions even in England. 
