26 GASTEROPODA OF THE INFERIOR OOLITE. 



Details of the Dobset-Someeset Disteict (No. 1). 

 The Mendip axis serves approximately as the northern boundary. 



Comparison with Normandy. 



There can be very little doubt that both the Upper Lias and the Inferior Oolite 

 of Dorsetshire present considerable analogies with beds of the same age in Normandy. 

 This, indeed, has been pointed out by Oppel and Waagen, the latter of whom 

 included Dorsetshire in the Parisian basin as distinct from the Inferior Oolite of 

 the whole of the rest of England. 



As regards the Upper Lias we have only to compare the general section at 

 Ilminster, given by the late Charles Moore, 1 F.G.S., with the section at Evrecy, 

 given by Prof. Eugene Deslongchamps 2 to perceive that above the margaritatus -beds 

 (Marlstone), there come theLejitana-heds, and then that remarkable zone of nodules, 

 or doggers, with Saurians, Fish, and Crustacea, succeeded by certain beds con- 

 taining Cephalopoda characteristic either of the Upper Lias proper, or of the 

 insignis-zone. At Ilminster the whole of this, judging from the above-quoted 

 section, is contained in about eight feet six inches, whilst at Evrecy the thickness 

 of the corresponding beds does not exceed thirteen feet. But now supervenes an 

 element of great physical difference in the Yeovil sands, over 100 feet thick, which 

 separate the bifrons-beds from the regular Inferior Oolite of Dorsetshire. It is 

 not a little singular that, whilst the life-zones remain almost identical in the two 

 countries, there should be no adequate physical representative of these Yeovil 

 Sands in the Department of Calvados. 



But whilst the life-zones present such a singular resemblance, their arrangement 

 and classification have been by no means uniform. Let us consult the work of 

 Deslongchamps already quoted, in order to see how variously the Inferior Oolite 

 may be divided or subtracted from according to the views of various authors. The 

 learned Professor of Caen then wrote that the " systeme oolithique inferieur " con- 

 sists of (1) Infra-oolitic Marls, (2) Inferior Oolite, (3) Fuller's Earth, (4) Great 

 Oolite. His systeme oolithique inferieur is in fact the equivalent of the whole of 

 our Lower Oolites plus the Upper Lias. I have drawn attention to the point 

 mainly in order to show the development of the lower portion of the Inferior Oolite 

 in Normandy, and its attachment to the Upper Lias, according to certain views. 

 For this purpose it will be necessary to analyse the " Infra-oolitic Marls " to see 



1 ' Proc. Som. Arch, and Nat. Hist. Soc.,' vol. xiii (1865-6). 



2 ' Etudes sur les etages jurass. inf.,' p. 75, 1864. 



