4 OKHAMANDAL MARINE ZOOLOGY PART II 



The shell of the adult chank is characteristically thick-walled and massive ; in the 

 live condition the exterior is covered with a dense brown and velvety horny layer, the 

 periostracum ; after death this dries, becomes brittle and eventually peels off, so that 

 shells exposed for any length of time on the sea-shore or buried for any considerable 

 period, become naked and reveal the characteristic snowy-white porcellaneous nature 

 of the shell clearly. In fully grown shells the rows of chestnut brown spots which are 

 normally very distinct on the outer surface in the immature, tend to disappear and often 

 become entirely obliterated. Around the mouth, especially along the inner edge of 

 the lip, a faint pink tint is often seen in fully mature shells, while occasionally the whole 

 interior surface of the mouth may assume a brick-red colouration particularly in shells 

 from certain localities. Characteristic of this genus are the three strong columellar 

 plications or ridges upon the columella ; the trace of a fourth is also frequently present. 



Local races. So considerable are the divergences in external form and colour shown 

 by shells of normal individuals of Turbinella pyrum that conchologists have made several 

 pseudo-species out of the principal variations. Intimate acquaintance, extending 

 over a period of upwards of ten years, with all the chank-producing localities on the 

 coasts of India and Ceylon, together with the advantage of having scrutinised several 

 hundred thousand shells, convince me that all the so-called species of Turbinella found 

 in Indian seas are referable to a single species T. pyrum and constitute, at the most, 

 varieties of the nature of emphasized local races. 



Three types exist which may be conveniently described as the obtuse, the central 

 and the elongate. All three forms have their own particular geographical distribution 

 and characteristic physical and biological environment. Normally they respectively 

 occupy well-defined localities. Intermediate forms linking the different main varieties 

 frequently occur, especially where the habitats of two varieties march, forming a debat- 

 able land where it becomes difficult or even impossible to assign the shells there found 

 to one or other variety, so evenly are the respective characteristics balanced. But the 

 vast bulk of the shells fall readily into one or other of the three types. 



(a). The central variety preponderates largely over the other two ; both numeri- 

 cally and industrially it is the most important and most valuable, being the form specially 

 valued by the shell-bangle manufacturers of Bengal for the purposes of their trade. 

 This type forms the bulk of the immense number fished off the Indian coast of the Gulf 

 of Mannar and around the north of Ceylon, a number averaging some 1| million 

 annually. Its range on the Indian 'coast is straitly limited on the north, where 

 Pamban Pass and the north coast of Rameswaram Island at the head of the Gulf of 

 Mannar mark the boundary, thence southward it extends along the shores of the 

 Ramnad and Tinnevelly Districts. North-westward of Pamban Pass this variety 

 marches with that which I shall term obtusa. Off Ceylon this central form is 

 restricted to the northern coasts from the Pearl Banks and Mannar round to 

 Trincomalee on the north-east coast. 



