CHASTER : STFXIES AND A'ARIATION. 29 



symmetrical so that, were the embryo divided along the middle line, 

 the two halves would correspond perfectly with one another. As 

 growth advances however, certain organs tend to pass more and 

 more to one side whilst others fall to the opposite side, so that they 

 may be adapted to the spiral arrangement. In the course of 

 generations it comes to pass that the process takes a definite 

 course in each species so that the resulting shell is normally dextral 

 or sinistral. Occasionally, however, a lapse occurs and a reversed 

 specimen results. 



There are numerous monstrosities recorded in which the soft parts 

 are the ones presenting the abnormality. Jeffreys^ states that Mr. 

 Duprey found in Jersey a Littorina obtusata which had three 

 tentacles and three eyes. The middle tentacle was bifid ; and the 

 eye belonging to it was double and presented two small points in 

 juxtaposition. The other eyes were simple or regular. 



Albinism is a condition dependent upon an innate inability of the 

 animal to produce pigment. In conchology it must be borne in 

 mind that the lack of colour in the shell is no more evidence of 

 albinism than is the white skin of a European. In the true albino 

 the normal colouring of the animal is absent, even the eyes being 

 colourless. Several years ago I discovered a large colony of Acme 

 litieata near Penmaenpool in Merionethshire, all the individuals of 

 which were true albinos, none of the animals showing any trace of 

 colour except in the liver, the colour of which belongs to a different 

 class from that of a pigment inasmuch as it has not been produced 

 for the sake of its colour but is dependent on and associated wath 

 the functions of a gland. 



Atavistic reversion or the sudden re-appearance of a character 

 which was possessed formerly by the species, but which has long 

 been lost, is an occurrence that is so well known and so frequently 

 observed in other groups that we may very safely assume that it 

 occurs in the mollusca. 



Individual abnormalities or sports — as they are called by 

 gardeners — are remarkable forms that occur sporadically. The 

 cause of their occurrence seems to be altogether unknown and 

 inexplicable. Scalariform shells belong to this category. I have 

 brought three specimens of LinincBa peregra which were all taken 

 one summer in a small pond amongst the Birkdale sandhills. These 

 serve to illustrate admirably different examples of these wild growing 

 forms.. Had they been captured in some little known country 

 each might have been considered the type of some new species 

 or genus, new species which would very likely remain for all time 



1 Pioc. ZooL Hoc, 1883, p. 112. 



