28 GASTEROPODA OF THE INFERIOR OOLITE. 



throughout by sharp-keeled Ammonites, and contains an assemblage of Gasteropoda, 

 which present considerable differences from those of the Upper Division. Such 

 are its Liassic affinities that, as we have already seen, Eugene Deslongchamps for- 

 merly placed it at the head of his " marnes infra-oolithiques," whilst D'Orbigny 

 assigned its fossils here to the Toarcian, there to the Bajocian, although in reality 

 the species of the two horizons are for the most part distinct. 



4. Zone of " Am. primordialis." In Normandy this is described as not usually 

 much more than three feet thick, often less ; it reposes conformably in most places 

 on the marls with Am. bifrons. In some places occur a great quantity of Gaste- 

 ropods and Bivalves having a facies thoroughly Oolitic ; often in a poor state of 

 preservation. Rhynchonella cynocephala is quoted from these beds. 



In everything but thickness this horizon probably represents the Yeovil Sands 

 with Am. radians ,Hm. Moorei, &c. (the Gloucestershire Cephalopoda-bed). 



Locality and position of the principal beds containing Gasteropoda. 



There are certain familiar names, well known to collectors of Inferior Oolite 

 fossils, frequently to be seen on the specimens themselves in our museums, &c. 

 As regards Cephalopoda, and especially Belemnites, these are so universally dis- 

 tributed, each group in its own order of succession, that in some places the entire 

 section of the Inferior Oolite might be termed a Cephalopoda-bed. The same 

 might almost be said of the Conchifera and Brachiopoda. The remains of the 

 Gasteropoda, besides being much more difficult to procure, have rather a tendency 

 to be confined to certain horizons ; not absolutely, however, as a few specimens 

 will, of course, be scattered throughout. The object of the present notice is simply 

 to draw attention to the locality and position of certain beds remarkable for the 

 number of Gasteropoda which they contain. In so doing the description of each 

 section will be given with a view more especially to emphasize the geological 

 position of such beds. Attention will be paid to the ordinary divisions where 

 that is practicable, but without attempting any of those refinements of generalisa- 

 tion which have been deemed possible by Oppel, and still more so by his successor 

 Waagen. I have already insisted on the fact that the Inferior Oolite, at all events 

 in the Dorsetshire district, is capable of being placed under two well-marked 

 divisions, and in each section the position of the dividing line will be indicated as 

 carefully as the facts known to me may justify. For general purposes the following 

 may be accepted as the zones of the Inferior Oolite, which have been traced, 

 though never all in one place, throughout the Dorset district. 



1. Parkinsoni-zone -\ 



2. Humphriesianus-zone > Upper Division. 



3. Sauzei-zone J 



