MARINE MOLLUSCA. 91 



particularly specified ; but bivalves are by no means deficient, 

 and some species (such as Venus clathrata and Area Okeni) are 

 characteristic of this stage. 



3. Marine animals of the fourth or Helvetian stage. 



During the formation of the three lower stages of the Swiss 

 Miocene, the sea only touched the frontiers of the country, or 

 traversed its low lands in separate narrow arms ; in the Helvetian 

 period, however, it had a wider extension, as has already been 

 shown (vol. i. p. 293) . The deposits formed at that period occur 

 in two zones, known as the Shell- sandstone and the subalpine 

 Molasse. It is a question whether or not the formation of these 

 two zones took place at the same time. The Shell-sandstone 

 which occurs along the Jura has been regarded sometimes as 

 older, sometimes as younger, than the marine Molasse which 

 follows the chain of the Alps. As the conditions of deposition 

 furnish no satisfactory data for the solution of this question, the 

 decision must only rest on the fossils. The Mollusca have 

 been very carefully investigated for many years by M. Carl 

 Mayer; and the list prepared by him shows, for the Shell- 

 sandstone, 218 marine Mollusca, for the subalpine Molasse 360 

 marine Mollusca, and for both together 421 species ; common 

 to both deposits are 141 species, so that the Shell-sandstone 

 shares about two thirds of its species with the subalpine Molasse. 

 There are 77 species of the Shell-sandstone wanting in the 

 subalpine Molasse ; but of these only 18 are peculiar to the Shell- 

 sandstone, 53 of these species being found elsewhere in the Hel- 

 vetian or in more recent stages, and 6 species being common to 

 the second or third stage. If the above-mentioned 53 species 

 are reckoned among the common forms as having been found 

 elsewhere in formations of the same age, nearly nine tenths of 

 the species will be found in common ; and there are only 24 

 which have not yet been observed in the Helvetian stage either 

 in Switzerland or in other countries. From this agreement of 

 the species alone, it would appear that the Shell-sandstone and 

 the subalpine Molasse belong to the same epoch; and this is 

 equally shown by the relations of the two faunas to the existing 

 race of animals. Of the 218 species of the Shell-sandstone, 76 



