Dr. PiTTON on the Strata below the Chalk. 



173 



longer diameter and their flatter surfaces in the horizontal position. Their 

 appearance when first uncovered by the removal of the rock above, was 

 that of elongated and flattened elliptical bodies, tapering at both extremities; 

 as represented in fig. 1. of the subjoined wood-cut. They consist of two 

 distinct portions ; a stony nucleus, of a dark brownish grey colour, with a 

 very slight tinge of purple ; and a crust or case, in the state of lignite, 

 which has externally a nearly uniform surface, and varies in thickness, in 

 different specimens, and in different parts of the same specimen, from 

 about -jV to Y an inch. This crust becomes thicker near the extremities of 

 the nucleus, and, in many instances, extends considerably beyond it, in a sort 

 of appendage at both ends ; but in other specimens of smaller size these 

 appendages do not appear to have existed. The annexed wood-cut explains 

 the structure of one of the largest specimens, and shows the relative propor- 

 tion of the parts ; and the figures in Plate XIX. show some of the principal 

 varieties of form which the fossil has hitherto presented. 



Fig. 1 



Fw. 2. 



_, Feet. 



Fig. 1 of the wood-cut on this page represents the external appearance of the entire fossil, as 

 seen from above ; the situation of the nucleus being indicated by an obscure dotted line. Fig. 2 

 is a longitudinal and vertical section, on the axis or line a, b ; and fig. 3, a transverse vertical 

 section, on the line c, d ; showing the proportion of the coating to the nucleus. 



The figures in Plate XIX., which are on a scale of one sixth, in diameter, of the natural dimen- 

 sions, are intended to represent some of the forms of the complete fossil, and of the nucleus when 

 divested of its coating of coal. They are, however, from necessity so much reduced, as to con- 

 vey but a very imperfect impression of the bulky objects which they represent. 



Fig. 1 represents the greater part of a very large nucleus, of which only one of the extremities 

 seems to be wanting. The remaining extremity is of irregular shape, as if it had been broken ; 

 and the specimen is much compressed in the middle. Fig. 2 is a nucleus apparently complete, 

 tapering towai'ds one extremity, nearly as may be supposed to have been the case in the part 

 which is wanting in Fig. 1. Fig. 3, a and b, are horizontal and side views of a much smaller 

 specimen, still retaining its coaly covering, with part of the smooth surface, hereafter mentioned. 

 Fig. 4 is a larger specimen, with some of the sandy clay in which it was imbedded still adhering, 

 the laminae of which are curved as if they had adapted themselves to its form. The specimen 



