No. 6 (1921) COMMON MOLLUSCS OF SOUTH INDIA I95 



deeply tinted Soletellina ; the occurrence of this peculiar colour in 

 molluscs that live completely buried in the sand is evidence that 

 colour may be often of no significance in the economy of an animal 

 save as the visual evidence of the presence of some waste excre- 

 tion. A curious oblique ridge or rib extends on the inner surface 

 of each valve from the beak across the shell to the ventral margin. 

 In no other family is any similar structure seen. 



The habitat and shell structure of the BORERS {Pholadidac) 

 exhibit peculiar features of great interest. In countries where the 

 coast rocks are comparatively soft, these molluscs excavate burrows 

 in the same style as the Date-shells do in coral. In India, the 

 Borers are seldom found in rock — our gneiss and schist are too hard 

 for them ; instead they affect stiff peaty and clayey deposits where 

 boring is comparatively easy. The finest instance of this is on 

 the north coast of Palk Straits, to the west of Point Calimere. 

 Here a stiff clayey deposit is found ; the conditions must be ideal 

 for burrowing and food must be plentiful, for here a truly magni- 

 ficent shell, the Oriental Borer (Dactylina orien talis), is found in large 

 numbers and of a larger size than any European species. A length 

 of 4 to 4^ inches is common. The shell is beautifully proportioned ; 

 long and tapered behind, snowy white in colour, papery in appear- 

 ance, brittle but exceedingly hard. The surface is covered with 

 prickly sculpture, rasp-like in arrangement. There is no true 

 hinge, and the ligament has given place to an accessory shell plate. 

 More peculiar still, the dorsal edges of the valves have grown 

 outwards in such manner as to become reflected over the umbones 

 protecting them from damage. These umhoiuil plates are further 

 strengthened by supporting pillars and plates, dividing the space 

 under these strange outgrowths into a number of chambers. 

 Within the shell a short stout rod projects inwards from the edge 

 of the umbonar cavity. The Pholads appear to excavate their 

 burrows largely if not entirely by patiently rasping down the 

 walls by the semi-rotation of their shell, first to one side and then 

 to the other. The foot takes some part also in the operation. 



The valves gape at each end. From the hinder one project the 

 siphons, here united externally into a single organ, tubular in form. 

 The extremity, slightly bifid, projects beyond the mouth of the 

 burrow when the animal is feeding. The long narrow gills extend 

 into the inhalent siphon. 



