igo OUR COMMON BRITISH FOSSILS. 



moulting process appears to have peeled off the ex- 

 ternal hard shell in two or three pieces. Thus, the 

 head-piece, or cephalic shield, is usually found alone ; 

 the thorax, or ringed part, is also abundantly found 

 separate ; whilst the pygidium, or tail, is frequently 

 met with apart from the others, although it is usually 

 adhering to the thoracic part. Of course, animals 



Fig. 154. Ogygia Buchii. 



which have died and been buried in the mud are 

 found with all the above parts adhering to each other. 

 The carapace or shell differed in its character in 

 various species. In some it appears to have been 

 very thin, in others harder. It may have been more 

 or less chitinous, after the manner of the elytra of 

 beetles, strengthened by the presence of limy matter. 



