20 GASTEROPODA OF THE INFERIOR OOLITE. 
absence or feeble development of beds representative of these zones in particular 
places. The zones of Am. opalinus and Am. Sauzei must be regarded as of 
subordinate importance in this country. Moreover, even if the Fauna of a 
particular zone is present, the Ammonite which gives its name to the zone may 
not be present. In Dorsetshire such an absence rarely happens, because there 
we are dealing with an essentially Cephalopod facies, but in such deposits as the 
Lincolnshire Limestone, usually referred to the Sowerbyi sub-zone, it is not always 
easy to ascertain the horizon of beds in particular quarries. 
On the whole, I have concluded that the gaps which separate these four zones 
are of unequal value, and would propose to divide the Inferior Oolite into two 
main divisions. The lowest division includes the sub-zone of Am. Sowerbyi, the 
zone of dm. Murchisone, and the reputed zone of 4m. opalinus, all characterised by 
a well-marked group of keeled Ammonites. As regards the Cephalopoda it has 
affinities with the Upper Lias, or Toarcian. This lower of the two grand divisions 
of the Inferior Oolite might be known generally as the zone of Ammonites Mur- 
chisone in its extended sense, or we might simply call it Taz Lowrr Division oF 
THE Inrerior Oowtrp, and this is what I should propose to do in any tabular 
arrangement which may be ultimately adopted for illustrating the vertical distri- 
bution of the Gasteropoda. 
Our upper division has a very different Ammonite Fauna. The allied 
groups (genera, according to some) of Spheroceras, Stephanoceras, and Cosmoceras 
are the prevailing forms, though by no means to the exclusion of all others. 
Moreover this great change in the Ammonite Fauna appears to coincide with certain 
worn and bored surfaces, indicating a period when deposits ceased, and when, 
perhaps, a certain amount of destruction was effected, pointing to a change of 
physical condition and to a considerable lapse of time. I am not aware how far 
local geologists would bear me out in this assertion, but we shall have an opportu- 
nity for testing it more closely when we come to the details of the four main 
topographical divisions of the Inferior Oolite. Tau Urpnr Division or THE INFERIOR 
Ootits, therefore, comprises what is usually known as the zones of Am. Humphrie- 
sianus and Parkinsoni, which may be separated in some cases, but which apparently 
inosculate to a very considerable extent. It so happens that some of the very 
richest shell beds in Dorsetshire occur at the junction of these two zones, and 
their fossils have been assigned by some to the Hwmphriesianus-zone, and by 
others to the Parkinsoni-zone. Hence the advantage of a classification which 
does not attempt, in all cases, a division between these two. 
It will be remembered that in the Cotteswolds Dr. Lycett adopted certain species 
of Brachiopoda for his zonal arrangements, but, owing to the apparent absence of 
T. fimbria in Dorsetshire, this plan is not satisfactory. Roughly speaking our 
Lower Division would comprise the jimbria-stage of Lycett, with the addition of 
