186 Scicidijk lutdlii/encc — Mineralogi/ and ChciHistr>f. 



acid, and evaporating the solution over sulphuric acidj a crystallized 

 compound is obtained, which, according to the analysis of Anderson, 

 consists of sulphuric acid 57-3, molybdic acid 32.8, water and loss 9.9. 

 Two isomeric modifications seem to be indicated. — (Bcrzelius' Jahrcs- 

 Beiicht, 1842.) 



25. Calcareous Roch pierced by Helices. — M. Constant Prevost 

 exhibited to the Societe Philomatique de Paris, numerous spec'i- 

 niens of a very compact grey limestone, which appeared to him to 

 have been deeply perforated by Helices. He collected these speci- 

 mens himself, in 1831, on Monte Pelegrino, near Palermo, at an 

 elevation of about 200 metres above the level of the sea. He at 

 first supposed that the perforations were the work of marine litho- 

 phagous nioUusca, and that they indicated one of the levels of the 

 sea at a remote period ; but the irregular and sinuated form of the 

 cavities, — their depth (extending to 12 and 15 centimetres), — their 

 dimensions (being from 4 or 5 millimetres to -4 centimetres in 

 breailth),— and above all, the presence of a Helix of different ages, 

 belonging to the same species, and each individual lodged in a cavity 

 exactly proportioned to the dimensions of the shell, — led him to the 

 belief that the Helices had themselves scooped out their abode. The 

 diiHculty, however, of understanding- how they could accomplish this, 

 made him hesitate in announcing publicly the fact he had observed, 

 until new facts, and more direct and positive observations, had con- 

 firmed his opinion. He carefully collected fragments of the perfo- 

 rated rock, and the Helices which inhabited it. 



In 1839, when the Geological Society of France met at Boulogne- 

 sur-mer, M. Constant Prevost, along with Messrs Buckland and 

 Greenough, who attended the meeting, discovered perfoiations pre- 

 cisely analogous to those of Palermo in an equally hard limestone 

 in the neighbourhood of Boulogne (the mountain limestone), and Dr 

 Buckland, on breaking the perforated rock, found many Helices at 

 the bottom of the cavities. 



This new instance, although strengthening the presumption aris- 

 incf from the fact observed at Palermo, did not yet definitely settle 

 the question — Had the Helices pierced the stone, or had they merely 

 taken advantage of the old perforations of marine lithophagous mol- 

 luscs, and converted them into a residence ? At the meeting of the 

 British Association at Plymouth, in 1841, Dr Buckland remarked, 

 in reference to a Memoir by Mr Walker, on the destructive action 

 of Pholades, that all the perforations observed in calcareous rocks 

 arc not necessarily the work of marine molluscs, and he mentioned 

 Helices as likewise perforating stones, supporting this assertion by 

 (he observation made at Boulogne in 1839, and even adding that Mr 



