Yol. 59.] OF THE LOWER LIAS AT SEEBURy CLIFF. 399 



thickness of deposit to the north, and diminution of deposit to the 

 south. At Keynsham the deposits included in all three zones reach 

 a thickness of about 35 feet, at Sodbury of about 90 feet, 1 while at 

 Sedbury Cliff no fossils characteristic of beds higher than the upper 

 Psilonotus-zome were observed, noth withstanding the fact that the 

 thickness of the rocks above the Pleuromya-Be&s amounts to 30 feet. 

 This may perhaps be best explained by supposing a gradual de- 

 pression of the whole area round an axis, running nearly east and 

 west, somewhat to the south of the Eadstock area, and therefore 

 practically coinciding with the Mendip anticlinal axis. 



Although the beds composing these three zones at Eadstock are 

 almost entirely made up of limestones, it cannot be deduced as 

 a necessary consequence that the depth of the floor at Eadstock was 

 greater than in the area farther north, where the greater part of 

 the deposit is made up of shale. The similarity of the fauna 

 and the nature of the shale-partings, whether thick or thin, suggest 

 the practical identity of bathymetrical conditions throughout the 

 area. The preponderance of limestones towards the south seems 

 merely to imply proximity to a land-area, composed of limestone- 

 rocks, such as the Mendip ridge would naturally have supplied. 



2. The Relative Faunal Sequence at Sedbury Cliff. 



Owing to the inaccessibility of the upper beds, we were only able 

 to study in detail the lower 12 feet of Lias, but, since fallen 

 fragments of all the higher beds are to be found on the shore, there 

 is very strong negative evidence that no beds above the Psilonotus- 

 zone are represented throughout the 35 feet of Lias in the cliff, for 

 no fossils characteristic of the Angulatus-zoiie could be found. 



The accompanying range-diagram (p. 400) scarcely calls for ex- 

 planation. The continuous portion of any ordinate indicates the 

 beds throughout which the species is continuously abundant ; the 

 interrupted portions indicate those beds in which it either occurs 

 only sparingly, or which intervene between two zones of abundance. 

 The extremities of each ordinate simply mark the point at which 

 the species begins or ceases to occur in sufficient numbers for its 

 presence to be recognized without exhaustive search (so that an 

 exceptional early-arrival or late-survivor is disregarded) ; since we 

 are mainly dealing with species immensely prolific in individuals, 

 there is little difficulty in fixing the extremities of the range- 

 ordinates. 



As already remarked, the fauna of the lowest beds is almost 

 precisely identical throughout the area which includes Sedbury 

 Cliff, Sodbury, Stoke Gifford, Kelston, and Bristol; and, moreover, 

 the vertical distribution within those beds is remarkably similar. 2 



1 Proc. Bristol Nat. Soc. vol. x (1901) p. 22. 



2 Ibid. p. 3. 



