Vol. 59.] THE L0WEK LIAS OF SEDBUKT CLIFF. 401 



At Sedbury Cliff specimens of Ostrea liassica, Modiola minima, and 

 Pleuromya crowcombeia crowd the lowest 4 feet, and with them are 

 associated, in certain layers which occupy the same relative position 

 as at other points throughout the area, abundant specimens of 

 Cardium rhceticum and Unicardium cardioides. The strongly-ribbed 

 Lima valoniensis is also very common, and Avicula cygnipes occupies 

 its usual position near the very base; Avicula fallaoc is not common,, 

 and no example of Pholadomya glabra was discovered throughout 

 the section. 



The succeeding beds exhibit a considerable faunal change, for 

 Lima gigantea, L. Hermanni, and L. pectinoides 1 enter and imme- 

 diately become abundant, Pectens of the calvus- and Uxtorius- 

 types become common, while the shales are crowded with Anomia 

 and fragments of a Pseudodiadema. Of the forms which charac- 

 terize the zone of Pleuromya crowcombeia, only Ostrea liassica and 

 Modiola minima pass up into these beds, and here M. minima occurs 

 very sparingly, while 0. liassica is abundant only at the base. 



Hence it seems best to make the zonal division at this point, 

 rather than to carry the lower zone up to the first entrance of 

 Ammonites planorbis, which does not take place until some 4 feet 

 higher, at a level that marks no other important palseontological 

 change. The beds which contain Am. planorbis differ in no other 

 respect from those just below, and, in particular, the shales are 

 wonderfully uniform in their fossil contents, throughout the whole 

 series of beds above the suggested division. 



If this zonal division be adopted, the best index for the lower 

 zone is certainly Pleuromya croivcombeia, for the reasons suggested 

 by me in the paper on the Keynsham Lias already referred to. This 

 is by no means a new suggestion, for the term ' Pleuromya-Beds ' 

 for the beds of the Lias has long been in use. 2 



The whole of the beds above this division at Sedbury Cliff are, 

 then, best grouped together as belonging to the Psilonotus-zone, 

 notwithstanding the absence (or extreme rarity) of ammonites 

 from the lowest 4 feet. 



A point of some interest in the faunal sequence is the fact that, at 

 Sedbury Cliff, the first occurrence of ammonites, in any abundance, 

 does not occupy quite the same relative position as it does farther 

 south. 



If we compare the range-diagram given in this paper with that 

 given in the paper on the Lias of Keynsham, we see that, whereas 

 the relative ranges of the most commonly-occurring lamellibranchs 

 agree very closely in the two cases, Ammonites planorbis enters 

 relatively later at Sedbury and persists relatively longer. 3 



1 = L. hettangiensis ; see Proc. Eristoi Nat. Soc. vol. x (1901) p. 49. 



2 See Tate & Blake's ' Yorkshire Lias ' 1876, chapt. vi, pp. 39-45. 



3 I here use the name Ammonites planorbis to imply the smooth form of 

 Am. psilonotus which has a moderate growth-rate, as distinguished from the 

 strongly-ribbed Am. Johnstoni with a slow growth-rate. The specimens of 

 Am. planorbis are ill-preserved at Sedbury Cliff, and especially so in the bed 



