20 GASTEROPODA OP 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 Am. Murchisonce, and the reputed zone of Am. 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- 

 chisona in its extended sense, or we might simply call it The Lower Division of 

 the Inferior Oolite, 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 Spharoceras, 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. The Upper Division op the Inferior 

 Oolite, therefore, comprises what is usually known as the zones of Am. Eumphrie- 

 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 Humphriesianus-zone, and by 

 others to the Parhmsoni-zorxe. 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 fimbria- stage of Lycett, with the addition of 



