Oliver. — Variation in Amphineura. 361 



Art. XXXIX. — Variation in Amphineura. 



By W. K. B. Oliver, F.L.S., F.Z.S., Dominion Museum, Wellington. 



[Read before the New Zealand Science Congress, Palmerston North, 26th January, 1921 . 

 received by Editor, 2nd February, 1921 ; issued separately, 8th August, 1921.] 



Variations in the shell-valves of the Amphineura from the normal number 

 of eight have been recorded from time to time. The following list con- 

 tains all the species I have been able to trace in which fewer than eight 

 valves have been noted. I have found no reference to specimens having 

 more than eight valves. 



Chiton tuberculatus, a West Indian species, was described by Linne as 

 having only seven valves (Syst. Nat., ed. x, p. 667). A second specimen 

 with seven valves was collected at Tobago. In this two of the valves were 

 soldered together, the result of an injurv (Pilsbry, Man. Conch., vol. 14, 

 p. 155). 



Mopalia ciliata, from California, with seven valves (Pilsbry, l.c , p. 305). 



Trachydermon cinereus, five valves, and Callochiton laevis, seven valves, 

 from British seas (Jeffreys, Br. Conch.', vol. 3, pp. 224, 227). ' 



Trachydermon ruber, six valves ; Ischnochiton conspicuus, six valves ; 

 and /. contractus, three valves. In the last example the reduction is 

 ascribed to the union of two or more valves (Sykes, Proc. Mai. Soc:, vol. 6. 

 p. 268). 



Plaxiphora egregia, six valves, and Sypharochiton pellisserpentis, five 

 valves, from New Zealand (Iredale, Trans. N.Z. Inst., vol. 40, 1908, p. 375, 

 pi. 31). 



Plaxiphora conspersa, with six valves, from South Australia (Bednall, 

 Proc. Mai Soc, vol. 2, p. 154). 



Onithochiton neglectus, with seven valves, trom New Zealand ; Cryptoplax 

 striatus, with three valves ; and Trachydermon cinereus, with six and seven 

 valves, from France (Pc-lseneer, Ann. Soc. Roy. Zool. Belg., vol. 50, p. 41, 

 1920). 



It appears, therefore, that the variations are always of the nature of 

 reductions, occur in various genera, and have been ascribed to injury, 

 union, or suppression of the valves. (See Pilsbry, Man. Conch., vol. 14, 

 p. xiii ; also Sykes, I.e., and Pelseneer, I.e.) 



The specimen I have no^ to describe differs from all those previously 

 recorded in that the reduction of the number of valves has occurred on the 

 left side only. Here is a case of meristic variation disturbing the bilateral 

 symmetry of the animal. A further result is to throw the median line of 

 the anterior, second, and third valves about 10 degrees to the left of the 

 median line of the remainder of the animal. The specimen was found 

 under a stone near low-tide mark at Shag Point, Otago, and is a member 

 of a species — Callochiton platcssa — rather rare in New Zealand, though 

 recorded from various points on the east coast between Stewart Island 

 and Rangitoto. The third and fourth valves are fused. The left side is 

 apparently normal, the lateral area belonging to the fourth valve, but the 

 rentral area may be derived from the third valve. On the right side 

 the third valve overlaps, but is fused to the fourth valve from the mantle 

 to the apex, which is double. The composite valve, therefore, consists 

 of the central and right lateral areas of the third valve fused to the right 

 and left lateral areas of the fourth valve. 



