KEPORT ON THE POLYPLACOrHORA. 5 



parts of the valve, number of slits, &c, even in closely allied forms ; thus care must be 

 taken not to lay too great stress on any one series of characters. 



The shell is more or less imbedded in and entirely surrounded by the girdle or zona, 

 the upper surface of which is variously covered with scales, spines, bristles, hair, or down. 

 The nature of the girdle forms a valuable auxiliary for systematic purposes, as the char- 

 acter of the covering is usually very constant in the different groups. In certain genera, 

 groups or even large tufts of spines or bristles are placed in pairs near the sutures between 

 the valves, and there are usually two on each side round the anterior valve, making a 

 total of nine pairs of tufts ; the tufts arise from depressions in the surface of the girdle, 

 which are termed pores : in Acanthochiton, for example, these pores are often very deep 

 pits. The terms which are employed to indicate the different kinds of girdle armature 

 merely have reference to their appearance, and have no morphological significance ; all of 

 them have a chitinous matrix, which is more or less completely infiltrated by calcareous 

 salts. The under surface of the girdle is characterised by possessing delicate elongated 

 flat scales. 



Another character which is worthy of notice is the number and extent of the gills ; 

 these are fewest in number in the Leptoidea, and reach their maximum development in 

 the Lophyroidea, Acanthoidea, and in the higher Ischnoidea. 



Professor Moseley has suggested the utilisation of the arrangement of the megalopores 

 and micropores of the shell for classificatory purposes. My researches on the structure of 

 the shells are not at present sufficiently complete to enable me to test the accepted 

 divisions of the group by this means. The histological structure of the tegmentum and 

 the distribution of the dermal processes (megalsesthetes and micrassthetes of Moseley) will, 

 however, prove of the greatest value and interest. 



I must take this opportunity of acknowledging my great indebtedness to my friends 

 Mr W. H. Dall, of the Smithsonian Institution, Washington, and to Mr E. A. Smith, of the 

 British Museum, who have at all times given me the benefit of their experience. To the 

 former I am further indebted for being permitted to examine Dr Carpenter's MS., which 

 Mr Dall has in his possession, and for the gift of several rare species of Chitons. 



The collection of Chitons was first forwarded to the Eev. E. Boog Watson, who had 

 named a few of them, and to whom I am indebted for some valuable clues. 



I have taken the following table from Dall's paper On the Genera of Chitons, to 

 serve as a key to the genera mentioned in this report, without however committing 

 myself to its entire acceptance, as it is better to leave the table as it stands than to make 

 alterations which would only be partial and incomplete. In the case of the Leptoidea 

 only have I given a synopsis of the genera of the group. Its introduction serves to 

 elucidate the concluding remarks concerning the probable line of differentiation within 

 the order. 



