THE BORING SNAIL. 55 



washed away. If the snail be removed and placed on Htmus 

 paper, the ruddy violet colour which at once tinges the paper 

 shows that there is acid of some kind, and if the paper be 

 applied to the spot whence the snail has been taken, the same 

 results follow. It is a remarkable fact that although the snail 

 leaves the usual slimy marks of its progress when crawling in 

 the summer time, no mucus is perceptible on the approach of 

 winter. When the cold months come round, the Boring Snail 

 leaves its food and attaches itself to the rock, remaining in the 

 same spot until summer approaches. During this time, the 

 portion of rock to which it clings is worked away, and the 

 stone around the excavation is impregnated with a greasy 

 matter which soon dries up after the admission of the atmo- 

 sphere. In a letter to me, dated October 14th, 1863, Mr. 

 Hancock remarks that the rock at Monte Pellegrino in Sicily, 

 which is crystalline and hard as marble, is perforated by the 

 same snail and in the same manner. I may here mention that 

 the stone of the Bois des Roches is that of which the column at 

 Boulogne is built, which has retained its sharpness of outline 

 after exposure to wind and weather for nearly sixty years. It 

 13 therefore called marbre Napolhfi Mr. Hancock proceeds 

 to say, ' The following are a few of the peculiarities which I 

 have not mentioned in my letter in the Field: 



' I St. There is no instance at Bois des Roches of a tunnel 

 being formed on the horizontal surface of a rock, or on the 

 sides facing the south and south-east. They are always on the 

 sides facing the north or north-east. 



* 2nd. The snail forms no epiphragm. 



[The * epiphragm ' is the barrier of hardened mucus with 

 which snails mostly close the entrance of their shells. There 

 are generally several epiphragms in each shell.] 



'3rd. Though during the summer it leaves behind it the 

 usual slimy mucus track; in the winter on returning to the 

 rocks no track is perceptible except the corrosion of the rock 

 by frequent passage. This would seem to point to a system of 

 secreting organs for the acid, separate from that for the mucus. 



* 4th. Contrary to the usual habits of burrowing molluscs, who 



