^usk-shells. Mail-shells, and Li?npets i 8 i 



The orders arc only four, but several of these are so 

 large that they have been divided into sub-orders. 

 The first of these orders {Ampliineura) consists of 

 the Mail - shells and some worm - like forms. Tlie 

 Mail-shells alone constitute the sub-order Polyplaco- 

 phora. The}^ differ from the vast majority of 

 Gasteropods in not being exactly univalve, for the 

 shell consists of eight overlapping segments or plates, 



Mail-shells 



the whole series surrounded by a muscular marginal 

 belt (the girdle) which partly retains them in position. 

 There is sufficient freedom of movement in these 

 plates to allow the Chiton, when alarmed, to roll up 

 after the manner of a Wood-louse. The under portion 

 of these plates is of the ordinary shelly material, but 

 the upper layer is chiefly of chitin. This layer is 

 the sculptured portion, and it is divided off into three 



