Mr B. N. Peach on the Occurrence of Pterygotus, etc. 347 



with the anterior portion the narrower. On the decomposi- 

 tion of the animal to which it belonged, the disjointed rings 

 seem to have been swept away by some current. On coming 

 to rest this particular one must have settled with its anterior 

 end downwards. This fact is proved by the rain pits on two 

 successive layers of the flag in which it is embedded. The 

 segment is also tilted over on one side as if by the current, 

 which, though no longer able to bear it forwards, yet, after 

 its lower part became embedded, helped to bend it as it 

 gradually became sodden, decomposed, and weakened. It 

 eventually became filled up inside, and covered up by the 

 accumulating sediment till it had been buried to a consider- 

 able depth. The material which forms the Caithness flags 

 is highly flaky, and it is to this that they owe their fissile 

 character. This sediment, then, must have fallen very lightly, 

 and become prone to be much reduced in bulk when sub- 

 jected to vertical pressure. The growing weight of sediment 

 had gradually compressed the layers among which our fossil 

 is embedded while they were still in a soft uncompacted 

 state, for the two vertical sides of the ring are highly corru- 

 gated, while the sloping sides are much less folded ; the angle 

 at which it has lain with the horizon must therefore have 

 been considerably diminished by the pressure (see diagxams 

 PL VII., Figs. 2 and 2a). 



" Adam's Plates " are produced when a closed segment has 

 been embedded in a vertical position, and then compressed. 

 In the case of one of the most perfect specimens in the 

 collection, which measures seven inches across, the particulars 

 of its formation seem to be this. As in the former instance 

 the disjointed tail ring has been borne along, and come to 

 rest with its anterior end downwards, to which fact the rain 

 drops on the flags are the witnesses. Though the ring was 

 undoubtedly very strong and chitinous at first, yet the pro- 

 cess of filling up and embedding seems to have been so 

 protracted, that the test had become weakened and bulged 

 out, and the inside flange of the posterior articular surface 

 had been somewhat flattened by coming to rest on the 

 material inside the ring. After being eventually embedded, 

 the pressure of the superincumbent debris had accentuated 



