No. 6 (1921) COMMON MOLLUSCS OF SOUTH INDIA 



123 



largest Indian gastropod, its shell weighing several pounds. In 

 shape it is not unlike a gigantic knobbed strombus but with the 

 outer lip thickened and the inner one spread iiorizontally over the 

 body whorl to form a flat polished surface in the fully adult con- 

 dition. These shells are rare and are found usually in 8 to 10 

 fathoms on the pearl banks of the Gulf of Mannar. A smaller 

 species, Cassis ni/a, growing to about six inches in length is found 

 in the Laccadive Islands. It is of a reddish tint, and the pro- 

 minent spines of C. coniiita are here reduced to low blunt knobs. 

 C. canalicidata is a common shore species of small size. 



The shell substance of the large Helmet-shells consists of 

 differently coloured layers, and this peculiarity is taken advantage 

 of by the cameo-workers of Naples and Torre del Greco, who 

 employ Cassis rufa for cameos requiring a dark red ground. Other 

 species give a purplish blue ground, the carved figure appearing 

 in white relief. 



The DoUulae are familiar objects on the Madras beach. The 

 shells have a widely expanded {ventricose) body whorl with a short 

 spire. Two genera are common, Doliiiiii, the TUN-SHELL, and 

 Pirula, the FiG-SHELL. Dolium is remarkable for its globose form, 



Fig. 14. Pirula seen crawling. Note the large flattened foot and the maitle 

 folds enwrapping the shell. (After Owen.) 



and the many narrow ribs that revolve round the shell from mouth 

 to apex. The tun is fragile but so light is it and so round that the 



