PLACOSTYLUS. 21 



memoir by Charles Hedley, who advances the theory that "the 

 various islands inhabited by Placostylus would seem to have been 

 joined, if not into one continuous and contemporaneous whole, yet 

 into larger fragments, which temporarily united, allowing the passage 

 of snails from one tract to another." He bases this conclusion upon 

 "the essential unity of the Placostylus area as a zoological province, 

 embracing the archipelagoes of Solomon, Fiji, New Hebrides, Loyalty, 

 New Caledonia, Lord Howe and New Zealand ; a unity explicable only 

 on the theory that they form portions of a shattered continent, and 

 are connected by shallow banks formerly dry land. This continental 

 area I propose to call the Melanesian plateau." 



In the dismemberment by subsidence of this land mass, it is likely 

 that the New Zealand, Lord Howe and New Caledonian area were 

 first divided from the rest, then the Fiji and New Hebrides areas 

 were segregated, leaving the Solomon group united to Papuan land 

 for a longer period. 



The Placostylus of New Zealand, Lord Howe and New Caledonia 

 are closely allied. Those of the other island groups are about equally 

 isolated one from the other, so far as this genus is concerned. Prob- 

 ably all the species of the Solomons are more closely related inter se 

 than any of them to those of other island groups, and the same applies 

 to the Fijian and possibly to the New Hebrides groups, excepting some 

 species of these islands which seem closely allied to New Caledonian 

 forms. For this reason, I have arranged the species geographically 

 in the following pages, following this sequence and classification : 



Archipelagoes. Sections. 



New Zealand and ) 



L -d H I ) P LACOSTYLUS Beck, typejibratus. 



(PLACOSTYLUS S. Stl*. 



New Caledonia. . | LEUCOCHARIS Pils ., type pmcher{ . 



f PLACOSTYLUS. 



New Hebrides. . -I PCECILOCHARIS Kob., type hartmani. 



(DIPLOMORPHA Anc., type layardi. 

 New Guinea. . . P remotus Hedley. 



( PLACOCHARIS Pils., type macyillivrayi. 

 Solomon Islands. < ASPASTUS Alb., type miltocheilus. 



( EUMECOSTYLUS Alb., type cleryi. 



(EUPLACOSTYLUS Crosse, type seemanni. 

 Viti or Fiji Islands. ( CALLISTOCHARIS Pils . ( type maUeatus . 



