98 SPECIAL CONVERGENCE 



left-handed, though the same species at the 

 present day is a right-handed one." 1 In India 

 and Ceylon, where the chank shell (Turbinella 

 pyrum] plays such a large part in religious 

 ceremonies, the rare apparition of a sinistral 

 form is regarded as a favourable portent, and 

 such a shell is very highly esteemed. There 

 would appear to be some common causes 

 operating during the early development to 

 procure such cases of reversal. A spiral shell 

 can only be twisted in one of two directions ; 

 and a Pleuronectid fish can only lie on one of 

 two sides. These alternative conditions, one of 

 which is frequently the rule, the other the 

 exception, afford an interesting sidelight upon 

 the incidence of convergence. 



The classification of reversed molluscs 2 is the 

 same in principle as that for reversed flatfishes, 

 namely : 



(1) Cases in which the genus is normally 



sinistral. 



(2) Cases in which the genus is normally 



dextral but certain species are nor- 

 mally sinistral. 



(3) Cases in which the species is indifferently 



dextral or sinistral. 



(4) Cases in which a sinistral form is an 



abnormal monstrosity. 



1 W. Bateson, " Materials for the Study of Variation," London, 



1894, p. 54. 



2 Cf. A. H. Cooke "Molluscs," Cambridge Natural History, 



1895, p. 249. 



