Vol. 59.] THE BOULDER-CLAY AT BIGGLESWADE. 37$ 



to be scarcely continued beyond the youthful stage, while in other individuals 

 the margin of this valve did not become free until more than one-third of the 

 adult dimensions were attained. The umbonal region is broad and thin, 

 flattened or with slight incurvation. The right valve is of flat or slightly- 

 concave form. A typical adult specimen measures 11 centimetres (or 4'4 inches) 

 in length and breadth. 



Ostrea discoidea is thinner and less massive than the large shells ascribed to 

 Gryphcea dilatata, Sow., which occur in the Elsworth Bock and Ampthill 

 Olay ; it also lacks the strong marginal thickening which characterizes the last 

 growth-stages of these. Although these Gryphceaz are usually further distin- 

 guished by a thick and massive left umbo, sharply truncated by the surface of 

 attachment of the youthful stage, individuals occur not infrequently which in 

 some measure exhibit characters suggesting a passage to Ostrea discoidea. 



Ostrea Iceviuscula, Sow., is distinguished from 0. discoidea by its much flatter 

 form, somewhat narrower outline, smoother surface, and narrow, pointed beak. 

 —July 1st, 1903.] 



Discussion. 



Mr. H. B. "Woodward observed that the thickness assigned to the 

 Ampthill Clay at Ampthill was about 60 feet. He agreed with 

 the Author that the transported mass had probably been removed 

 from an outlier somewhere to the north of Biggleswade. 



Mr. F. "W. Harmer was much interested in the paper just read. 

 This great boulder in the Chalky Boulder- Clay of Bedfordshire 

 might be compared with the enormous masses of Chalk seen in 

 the Cromer coast-section. The most important point of the paper 

 seemed to be that it tended to show that the movement of the ice 

 was from the north. 



Mr. E. T. Newton called attention to certain of the fossils which 

 had been found in the 67 feet of strata regarded as a portion 

 of ' Ampthill Clay/ and considered that these fully justified the 

 reference to that horizon. The presence of Boulder-Clay and 

 then Gault under the great mass was good evidence of its being a 

 transported block. 



Mr. Hudleston pointed out that, if the boulder was a portion of 

 the Ampthill Clay, we might have expected spines of Gidaris flori- 

 gemma in such a large mass ; also it was stated that fossils in that 

 clay were usually not pyritized, whereas in the boulder there is said 

 to be pyritization. Thus there were two reasons which rather 

 militated against its identification as Ampthill Clay. What was 

 the precise distance to the nearest undoubted exposure of Ampthill 

 Clay ? 80 far as he remembered, this clay was well shown on the 

 Midland Railway north of the long tunnel between Bedford and 

 Ampthill, but not at Ampthill itself. It was just possible that the 

 boulder might have been derived from the lowest portion of the 

 Kimmeridge Clay, and an examination of the specimens exhibited 

 rather strengthened this view. He further commented on the 

 remarkable thinness of the Gault, often 150 feet thick in that 

 district, as shown in the Author's section. Had the ice ploughed 

 away the higher portion, and left the Jurassic in the position of 

 the oldest uppermost? 



Dr. F. L. Kitchen, having complimented the Author on the lucid 

 manner in which he had presented the facts, made a brief allusion 



