268 I'KOCEEDINGS OF TIIK M ALACOLOGICAL SOCIETY. 



recorded in Spatha where the right valve overhipped the left.' The 

 teeth in Tapes are liable to vary, one tooth bciuo; often aborted or 

 lacking, and Venus macrodotz has been noticed with only one tooth 

 (not three) in one valve, and a corresponding cavity in the other. 



One curious abnormality noticed^ in the Polyplacophora is the 

 reduction in number of the usual eight plates, either by suppression 

 or union. Many years ago Montagu described a specimen as Chiton 

 septemvahis, and quite recently six-valved specimens of Traclujdennon 

 ruber, L., and Isclmocliiton conspicuus, Cptr., have been noticed. 

 Perhaps, however, the most remarkable is a three-valved specimen of 

 Ischiocliiton contractus, Rve., recorded by myself, in which the tail- 

 valve occupied a greater space than the other two valves put together, 

 being probably formed by union of two or more valves. This specimen 

 is now preserved in alcohol in the British Museum. 



Amongst Cephalopods we may refer to the interesting specimen 

 of Argonauta recently figured ^ in our Proceedings, which had a double 

 sinuation on the left side ; as also to that recorded by Adams,* which 

 had a portion broken out of the left side. He pointed out that this 

 was repaired in the usual manner, but that the fragment which had 

 been broken out, its loss having probably been prevented by the vela, 

 now formed two-thirds of the repaired portion, while it had actually 

 been reversed in position, the inside having become the outside. 



One of the most interesting phenomena, perhaps, is when the 

 coil of the shell in the Gastropods is reversed, accompanied by a 

 corresponding deformation in the animal, or when a similar occurrence 

 takes place in the Pelecypods. As is well known, the majoritj' of the 

 Gastropods have dextral shells, but a fair proportion are normally 

 sinistral. In some groups the bulk of the known forms are constantly 

 sinistral as in the ClausiliidiB, whilst m such as the Helicidae a few 

 species only have this character, or it may happen that certain 

 sections of a family are reversed. In other groups, again, such as 

 Amphidronms, Partxda, and Achatindla, some of the species are 

 normally found in both conditions ; the proportion, however, of each 

 kind will vary in each species, and it has been stated that Partula 

 otaheitana, Brug., is generally reversed, while P. vextllum, Pease, has 

 only one in fifty, and P. apnis, Pease, only one in several hundred 

 sinistral. Dyakia Lindstedti is another illustration of the princij)le that 

 a species may be normally both sinistral and dextral. We may also 

 recall the well-known case of Sipho antiquus, of which a colony of 

 sinistral specimens were found in Vigo Bay. 



Whether these forms be in a strict sense hereditary, it is hard to 

 say ; in my view the cause that rendered the parent reversed may 

 well, acting in conjunction with a predisposition to abnormality, give 

 a greater probability of reversal in the descendants. Nylander has 



' Jickeli: Naclirbl. Deutsch. Malak. Ges., vol. v, p. 69. 



^ Cf. Sykes: Journ. Malac, vol. vii, p. 164. Stearus: Nautilus, vol. xv, p. 53. 



Blaney: Proc. Boston Soc, vol. xxxii, p. 39. 

 3 Jide, vol. V, p. 310. 

 ■* Amer. Jouru. Sci., vol. vi, p. 138, 1818. 



