32 PROCEEDINGS OF THE M A LACOLOGICAL SOCIEXr. 



Plaxiphora biramosa (Qiioy «fe Gaimard) is quite unlike the pre- 

 ceding, tliough of large size. The exterior of the valves (which are 

 very solid) is sculptured, whilst the sutural laminse are connected, 

 a feature otherwise quite foreigti to the genus, and the posterior 

 valve is quite differently formed. I consider this species quite 

 worthy of sub-generic distinction, and I propose for it the new sub- 

 generic name Diaphokoplax. 



The group typified by P. costata (Blainville) is also easily diagnosed 

 by the medium size of the members, the sculptured exterior of the 

 valves, and the formation of the posterior valve. Study of Pilsbry's 

 Manual (loc. cit.) suggested the use of Euplaxipliora, Shuttleworth, 

 for this group, but reference to Bhutth-worth's paper (Mittheil. 

 naturf. Gesell. Berne, 1853) showed that this name was introduced 

 (p. 193) in such a manner that it can only be treated as a substitute 

 name for Plaxiphora, Gray, and is therefore unavailable. I propose 

 to designate this subgenerio group Poneroplax and to name Chiton 

 costatus, Blainville, as type. 



Fremhleya, founded upon P. egregia, H. & A.. Adams, has been 

 admitted as a section by Pilsbry and as a distinct genus by Thiele. 

 I would temporarily rank it as a sub-genus, the sculpture, small size, 

 and peculiar posterior valve being diagnostic. The commonest Plaxi- 

 phora in New Zealand is P. ccelata (Reeve), under which name more 

 than one species appears to be confused. The small size, definite 

 sculpture, and peculiar posterior valve determine this sub-genus ; the 

 tegmentum of the posterior valve ends in a pointed plane mucro 

 forming a triangle. I propose for this sub-genus, naming Chiton 

 ccelatus, Reeve, as type, Maorichiton. 



My conclusions regarding the division of the Australasian Plaxi- 

 phora are as follows : — 



Genus Guildivgia, Pilsbry. Type G. ohtecta (Pilsbry). 



,, Plaxiphora, Gray. ,, P. aurata (^Spalowsky). 



Sub-genus Plaxiphora, Gray. 



,, Diaphoroplax (supra). ,, P. biramosa (Q,. & G.). 



,, Poneroplax (supi'a). ,, P. costata (Blainville). 



,, /V(?/Mi/<'//«, H. & A. Adams. ,, P. egregia {K. & h.. KH.). 



,, Maorichiton (supra). ,, P. ccelata (Reeve). 



The question may be raised whether this subdivision is necessary 

 and will it be useful. To the first, I would point out that it tends to 

 exactness and certainly makes work more facile and identifications 

 more certain. To the second, I saj^ emphatically that it will be most 

 useful, especially to the zoogeographer, and 1 give the following 

 notes. The genus Plaxiphora is admittedly Antarctic in its 

 distribution. The typical sub-genus occurs in the Falkland Isles, 

 Southern South America, the Sub-Antarctic Islands of New Zealand 

 commonly, and the mainland more rarely. It does not occur in 

 Australia. The sub-genus Diaphoroplax is, so far as I know, confined 

 to New Zealand, as is the genus Ouildingia. The sub-genus 

 Poneroplax occurs throughout Australia, and possibly P. frembleii, 

 Broderip, should be referred here. The species " P.glauca, Q,. & G." 



