348 Proceedings of the Royal Physical Society. 



the bulging, and had corrugated the sides where they approach 

 the vertical. The flag, in being split open, has given way 

 along one bedding plane till the body ring is reached at its 

 vertical part. The line of the interior of the ring is then 

 followed backwards till the inner or truncated edge of the 

 flange is reached, whence the flag has split along another 

 bedding plane at a little higher level than the first. The 

 upper half is the only part of the specimen which has been 

 retained, and when this is turned upside down a circular 

 depression, like that made by a quoit being pressed into some 

 plastic material, is observable (PL VII., Fig. ?>-od). The 

 animal to which the above segment belonged could readily 

 have been a specimen of P. Anglicus. If so, the segment 

 must have been about the fourth or fifth from the telson, as it 

 bears no dorsal nor lateral keels. It is comparable to one of 

 that animal as to size, and in another respect it is like it in 

 having no ornament on the hinder portion of the segment. 

 A few squame-like embossings are observable where the sides 

 of the plate become vertical, but they are much obscured by 

 the puckering alluded to. 



There are several modifications of " Adam's Plates." One 

 ring has given way before being subjected to pressure, and 

 has been rent by several vertical tears. The fossil now looks 

 like the rim of a bottomless plate which has been broken up 

 into several portions by the dividing cracks which radiate from 

 what appears the centre of the would-be plate. Occasionally 

 a tail ring has split at one place only, and partially opened 

 out, but has still retained sufficient curve to cause it to pass 

 up through several layers of sediment, and so to become 

 corrugated during the compression. 



With the exception of one piece, which appears to be the 

 portion of an Endognath from which the toothed margin has 

 been broken off, every portion of Pterygotus in the collection 

 appears to belong to the hinder segments. It may be that 

 these rings were more conspicuous, and that they caught the 

 eye of the quarrymen who may have therefore preserved 

 them in preference to other portions ; or, what is quite as 

 probable, the remains had been water-borne for a considerable 

 distance before being embedded. With the exception of the 



