Art. XXXIX.—Variation in Amphineura.
[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 contains 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 Linné 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 injury (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 I. contractus, three valves. In the last example the reduction is ascribed to the union of two or more valves (Sykes, Proc. Mal. 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, pl. 31).
ciphora conspersa, with six valves, from South Australia (Bednall, Proc. Mal. Soc., vol. 2, p. 154).
Onithochiton neglectus, with seven valves, from New Zealand; Cryptoplax striatus, with three valves; and Trachydermon cinereus, with six and seven valves, from France (Pelseneer, 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, l.c., and Pelseneer, l.c.)
The specimen I have now 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 platessa—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 central 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.