No. 6(l92l) COMMON MOLLUSCS OF SOUTH INDIA 



i8i 



our common shells we can find a full gradation of transition 

 stages, beginning with instances where the siphonal openings, one 

 incurrent (to the gills) and one excurrent or anal, are formed by 

 the temporary coming together of the mantle edges of opposite 

 sides at one or two definite points, thus forming temporary open- 

 ings. The next stage is for these temporary junctions to become 

 permanent and then for the edges of the openings to lengthen out 

 gradually into tubes of varying length. The Tellinas aid their 

 near relatives Donax, Mesodesma and Psammobia are noteworthy 

 for the extreme length attained by their siphons. The shells are 

 usually small and as their habitat is in shallow water on sandy 

 coasts, seafowl and other birds search the sands for them at low- 

 water, to say nothing of predaceous fish that take up the hunt 

 whenever the depth of water permits. Hunted continually, these 

 molluscs lie in deep burrows with just the tips of their siphons 

 level with the surface — as inconspicuous as may be. If discovered 

 they withdraw the siphons into the shells lying below at a depth 

 of may be a couple of inches and begin with lightning speed to 

 burrow still lower by the help of their wedge-shaped muscular 

 foot. The longer the siphon, the greater the protection; it is 

 significant that the four genera named as having peculiarly long 

 siphons are the commonest bivalves along our sandy shores — 

 infinitely more numerous than those with short siphons. 



Two species of the WEDGE-SHELLS (Donacidae) known on the 



Madras Coast are the small Do/iax 

 cuneatus or MURAL (Tamil) and the 

 much larger Donax scortum. In 

 outline the former is a small much 

 compressed bivalve roughly 

 wedge-shaped ; the posterior part 

 of the shell obliquely truncate. It 

 is abundant between tide-marks 

 and for some short distance 

 below low-water level. It never 

 enters backwaters and is essentially a marine form. In size it 

 seldom exceeds 40 mm. in length. On the East Coast, the mural 

 is particularly plentiful on surf beaten sand-flats ; on the Ramnad 

 Coast the name mural gives place to -vazhiiiiatti. Along the 

 Malabar shore it is less plentiful and is usually smaller. In 

 Tamil districts, especially on the Coromandel Coast, the mural is 



1-"IG. 47. The Mural {Poiui.x cii-iu' Tins). 



