BURROWING MOLLUSCS. 67 



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

 results follow. It is a remarkable fact that althoxigh 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 atmosphere. 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 man- 

 ner. I may here mention that the stone of the Bois de Eoches 

 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 is therefore called marbre 

 Napoleon. Mr. Hancock proceeds to say, " l"he following are 

 a few of the peculiarities which I have not mentioned in my 

 letter in the Field : 



1st. There is no instance at Bois de Eoches 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 

 generally have a bed of muddy matter between their shells and 

 the walls of their dwelling, the Helix saxicava keeps his tunnel 

 perfectly clean and neat. 



5 th. When the liquor alluded to as forming a fatty aureole 

 round the tunnel penetrates into pre-existing clefts in the rock, 



B 



