No. 6 (1921) COMMON MOLLUSCS OF SOUTH INDIA 109 



pushing upwards to the surface of the mud the end of a^ long 

 muscular tube. Through this air is passed down to the " lung 

 cavity." Pachylabra is also prepared for the complete drying up 

 of the mud during the dry season ; it withdraws its whole body into 

 the shell, closes the opening behind it with its strong calcareous 

 operculum and remains dormant till the rains come, when the caked 

 mud enveloping it softens and becomes liquid once more. The 

 eggs are large and attached together in masses. In the Tanjore 

 and Tinnevelly districts it is regularly collected after the paddy 

 has been harvested, by poor people who^use it to eke out their 

 ordinary meals. It is also used medicinally as an application for 

 sore eyes. In the Philippine Islands it is a regular market 

 commodity. 



Fig. 4. Life appearance of Pachylabra cariiiata. It has climbed to the surface of the 

 waler up the glass front of an aquarium and is inhaling air. Note the large 

 expanse of foot and the wide i]ihalent siphon. 



The Periwinkles {Littorinidae) are found on rocky shores in all 

 parts of the world. They are small turbo-like shells, but differing 

 therefrom in having no pearly lining ; the operculum is horny. 

 They appear closely related to the fresh-water snails (Vivipara and 

 Pachylabra), for they can live a long time out of water especially 

 on coasts where the rise and fall of the tide is great. In some 

 species this sub-aerial habit is so highly developed, that indivi- 

 duals may be found living so high on the littoral as to be reached 



