CHITON. 209 



lusca and the Mammalia had their own respective modes 

 of organization, and that the theory above mentioned was 

 by no means justified by the facts. Each plate of a Chiton 

 has its sides diagonally parted, and is divided into three 

 triangular areas. The base of the central area is covered 

 by the edges of the preceding plate, and the base of each 

 lateral area is inserted in the girdle or marginal band of 

 the mantle. The front plate, being that which protects 

 the head, is semicircular ; the hindmost plate is oval, 

 and is furnished with a boss or point, which overhangs 

 the rear and corresponds with the apex of the cone in the 

 shell of Patella. In many species the plates are inserted 

 more firmly in the girdle by means of marginal notches. 

 These were first noticed by Fabricius in his description 

 of C. marmoreus. They vary in number and fineness 

 according to the species. The spines and valves which 

 cover the girdle in most species are calcareous. The 

 structure of the shell agrees with that of Patella, 

 although the details are somewhat different. Carpenter 

 says, ' ' The external layer, which is usually impregnated 

 by colouring matter, does not exhibit the laminations 

 which are seen in Patella, but in their stead presents 

 everywhere a delicate fibrous structure, the fibres being 

 arranged parallel to the surface. The superficial part of 

 this layer is perforated by large canals, which pass down 

 obliquely into its substance, without penetrating so far 

 as the middle layer. The purpose of these canals, which 

 remind us of the perforations of Terebratula, is by no 

 means apparent. In the deeper part of this coloured 

 external layer, which is of great toughness, there is a 

 layer of minute cells which seem to lie between the 

 fibres ; and below this, again, is a layer entirely com- 

 posed of large flat pavement-like cells, as in Patella. 

 The internal layer seems to have the same nearly homo- 



