POLYPLACOPHORA. XX111 



CLASSIFICATION OF CHITONS. 



The classification given in this work is based mainly upon char- 

 acters furnished by the articulating surfaces of the valves. The 

 hard structure in this group is comparable to the vertebrate skele- 

 ton in the great variety of stresses to which it is subject. Each 

 valve is not only acted upon by external forces and its body-mus- 

 cles, but by the valves before and behind it, and by the girdle. 

 Hard parts of great complexity have thus arisen, offering to him 

 who can decipher their story clues as valuable for the construction 

 of genealogical lines as have been found in the limb-bones of mam- 

 mals, or the hinge-teeth of bivalve mollusks. The development of 

 the gills, foot and girdle has been given due weight, and for the first 

 time, systematic use has been made of the sense organs of the shell 

 (tegmentum), and the system of sculpture of the latter. The non- 

 differentiation of the dentition throughout the group has prevented 

 the use of that character usually so important. 



The Polyplacophora known to us, form a group remarkably homo- 

 geneous, when we consider the vast antiquity of the stock. The 

 fossil forms are yet few in number and imperfectly known ; so that 

 all attempts to trace the mutual relationships and descent of modern 

 genera must still be based upon the morphology of living forms. 

 The complex inter-relations of the various families and genera, the 

 number of phyla, and especially the wonderful parallel develop- 

 ments of different stocks, all render a linear arrangement of groups 

 highly unsatisfactory. The accompanying diagrams show the main 

 facts of classification and descent much more clearly. 



It is commonly known that the earlier (Palaeozoic) Chitons are 

 without exception, destitute of plates of insertion, and belong, 

 therefore, to the family Lepidopleuridce. This family has survived 

 to the present day, but the species are now few in number and of 

 small size. The higher genera of Lepidopleuridce possess insertion 

 plates on some or all valves, but they are unslit ; and it is to this 

 branch that the genesis of the higher Chitons is to be traced. The 

 lowest Isehnochitonidce known have already become well differenti- 

 ated by the development of a definite system of slits. 



The Ischnoid stock must be regarded as a synthetic or unspecial- 

 ized type of Chitons. In other words the common ancestors of the 

 other families of the Chitons (except Lepidopleuridce), would proba- 

 bly, if known to us, be classed as Ischnochitonidce. From this general- 

 ized Ischnoid stock specialization proceeded in two directions : (I) 



