SCREWSTONES. 



103 



spiral side-passages, each of which served as a dwelling-place for 

 one animal. The tubes were afterwards filled up, and thus these 

 singular " screwstones " (fig. 326) were produced. In favour of 



Fig. 327. 



Fig. 326. 



Fig. 325. 



Fig. 325. Gordiopsi-s valdensis, Heer, from the shales of Troistorrents, in 



the Val d'llliers. 

 Fig. 326. " Screwstone " from the Martinsbruck, in the Canton of St. 



Gall, half nat. size. 

 Fig. 327. Helminthoida molassica, Heer, from Reiden. 



this explanation, the fact may be noticed that near the Martins- 

 bruck in St. Gall, where these sandstones are particularly fine, 

 M. Carl Mayer has found a Lutraria (L. sanna) in one of them. 

 Near Rorbas, according to Dr. Biedermann, the screwstones are 

 found in the uppermost layer of the lower freshwater Molasse at 

 the boundary of the marine Molasse, which has furnished the 

 materials for them; these animals consequently bored their 

 holes in the hardened soil. 



The Glycimerida, Solenacea, Pholadida, and Teredina, belong- 

 ing to the same great division, form their dwellings, in like 

 manner, sometimes in the sandy bottom' and also in wood and 

 rock ; and in earlier periods of the earth's history they had the 



